On the planet Világ, the talk show Galen Today hosts a heated debate on the subject of research into Killoran technology. The joint heads of a campaign opposing such research, Dr Sofia Rossiter and Sebastian Lawrence, face off against their political opponent: Sofia’s stepmother, Evelyn Rossiter. Evelyn has lost her temper once already, and can barely restrain herself as Sofia and Sebastian claim that independent polls have indicated that nearly 89% of the population wants the alien technology destroyed rather than integrated into their own. Evelyn claims that the research is being conducted by an independent body with no ties to the government, but in fact she and her husband, Principal Triumvir Rossiter, are funding much of the work themselves. Evelyn finally loses her temper again, lashes out at her stepdaughter’s narrow-minded ignorance, and storms violently off stage with a severe migraine, leaving Sofia and Sebastian to take the moral high ground.

Elsewhere, the Doctor has once again saved the day and the lives of many an Earth colonist, and he’s puffed up with a sense of his own importance about it. Mel, irritated, advises him to tone down his attitude, but the Doctor reminds her that he’s a much more mellow person than he used to be, thanks to the steadying influence of one of his past companions. Mel’s heard all about Evelyn Smythe before, but they’ve never actually met -- and the Doctor thus impulsively decides that the time has come to visit his old friend again and introduce Mel to her. He had told Mel that Evelyn left him to run off with a man, but as he sets the co-ordinates for Világ, he’s forced to admit that he may have embellished the story somewhat...

Sofia is disgusted with Evelyn’s behaviour, convinced that her stepmother’s arrogant behaviour is making her father look like a fool. Sebastian assures Sofia that the people still respect her father, and that they respect her, too; Sebastian started his own medical career because of her, and the brilliant Dr Andrew Szabo has even claimed that she will outstrip his achievements one day. This reminds Sofia that she’s supposed to meet Szabo for lunch. As she heads off, she mutters to herself that her life would be much better without Evelyn in it -- and unfortunately, Sebastian seems to take these casual words very much to heart. Unaware of the plans Sebastian is laying in her absence, Sofia joins Szabo for lunch, still fuming over the incident on the talk show and dreading this afternoon’s memorial service, at which she’ll have to sit and listen to Evelyn give a eulogy for Krisztina, Sofia’s late best friend. Szabo, very much a father figure, soothes Sofia’s jangled nerves, but admits that he won’t be at the memorial himself, as there are still people who object to what Szabo did after the war.

Elsewhere, Rossiter dresses for the memorial ceremony, while listening to a news report on the increased rates of domestic violence in the city and the explosive discussion on Galen today. Evelyn then storms into the room, still fuming over the incident and ready to snap at her husband, but he’s used to her fits of temper and manages to calm her down. She apologises for her behaviour and assures him that she’s taken her medicine, although her headaches do seem to be getting worse. He puts this down to the stress of her political position, and assures her that he still loves her as much as the day she returned to him. He advises her to slow down, however, reminding her that she’s recently had exploratory surgery, even though Szabo had her checked out of the hospital within a day. Evelyn promises that she’ll try to relax, and as she and her husband prepare for the ceremony, she remembers the day that she returned to Világ...

Two Years Earlier: The Doctor and Evelyn return to Világ one year after the Killoran invasion. Reconstruction is still underway in the city of Galen, and to the Doctor’s irritation, he and Evelyn are kept waiting when they try to visit Rossiter. Rossiter’s daughter Sofia arrives to greet them, but coldly informs Evelyn her father is not receiving visitors. Upset by Sofia’s unfriendly attitude, Evelyn remains calm and polite, but makes it quite clear that she’s not leaving without seeing Rossiter -- unless Sofia is willing to have security throw her out. As Evelyn stalks out to locate Rossiter herself, the Doctor tries to placate Sofia, but the young woman remains upset that her father’s new love has returned, and the Doctor can do little but return to the TARDIS and hope it all works out. Meanwhile, Evelyn finds her way to Rossiter’s office, but pauses outside, unsure whether he’ll be happy to see her again -- and as she hesitates, Rossiter himself emerges from his office, swamped in paperwork, and all but walks right into her.

Evelyn returns to the TARDIS, where she tells the Doctor that Rossiter is doing well and that Suskind, who has mellowed considerably since the Killoran invasion, actually helped to draft the new triumvirate government’s constitution. She then informs the Doctor that she’s decided to leave him and stay on Világ with Rossiter; this was always in the back of her mind, and once she met Rossiter again, they picked up exactly where they’d left off. The Doctor shows no particular surprise, and is in fact rather brusque as he bids Evelyn farewell. A little upset by his attitude, she asks him to stay long enough to speak with Rossiter, but the Doctor clears his throat and claims to have urgent business elsewhere. Evelyn sadly takes her leave of the Doctor, inviting him to return one day, but he makes no promises, and once she’s out the door, he dematerialises, bringing their close friendship to a rather abrupt end.

The leader of the kidnappers reveals himself to Mel and Evelyn as Sebastian Lawrence, and angrily punches Mel when she calls him a thug. Back at the park, the Doctor, Sofia and Rossiter recover from their exposure to the gas, which the Doctor identifies as a mild anaesthetic -- something commonly found in hospitals. Sofia bristles at the implication, and insists that, despite her antagonism towards Evelyn, she would never condone something like this. Security forces arrive somewhat belatedly, but the Doctor stops Rossiter from rushing off half-cocked in search of his wife, insisting that he instead let security do their job and get him and Sofia to a place of safety. The kidnappers then deliver a message threatening to kill Evelyn unless all Killoran technology on the planet is destroyed, but the Doctor reveals that he can use the TARDIS to trace the source of the transmission. Rossiter insists upon accompanying him, despite Sofia’s objections. Jenner has recovered from his injury, a flesh wound to the arm, and Rossiter orders him to Sofia to the hospital and to guard her with his life. Once the Doctor and her father have gone, the worried Sofia tries to contact Sebastian -- and when she fails, she orders Jenner to requisition a blade shuttle, hoping that her suspicions are wrong.

At Sebastian’s safe house, Evelyn tends to Mel’s injuries and explains the political situation. She concedes that Sofia and Sebastian aren’t entirely in the wrong, but she’s unsure whether Sofia is acting out of concern for her planet’s well-being or whether she just started the campaign out of spite against Evelyn. The clearly distressed Sebastian then enters to check on his hostages, but Evelyn refuses to accept his apparent contrition; however, as Sebastian angrily storms out again, Evelyn tells Mel that she’s just taking a page from the Doctor’s book by refusing to let the villain get the upper hand. Mel realises that Evelyn has fond memories of her time with the Doctor, and Evelyn tells Mel about seeing Daleks, mutant vampire hunters, and even, eventually, Elizabeth’s court. She casually asks why the Doctor returned now, after so long, and seems disappointed to learn that it was just to prove a point to Mel. Evelyn then suffers a migraine, and when Mel tries to give her a glass of water, Evelyn angrily slaps it from her hand. She then apologises for her strange shortness of temper, and admits that she was suffering from a heart condition when she travelled with the Doctor; however, she visited Dr Szabo six months ago and had it taken care of. Mel wonders if Evelyn regrets leaving the Doctor, but Evelyn claims to be happy here; in any case, they have more pressing concerns. Sebastian is clearly just holding on to his emotions, which means they can easily provoke him into making a mistake -- and this room’s door can be locked from the inside.

As the Doctor and Rossiter head for the TARDIS, Rossiter warns the Doctor that Evelyn’s temper has been hot recently, perhaps because of all the political pressure she’s been under, and fears that she may do something to provoke the kidnappers. He admits that Sofia still remembers her late mother fondly and feels betrayed that her father has remarried; nevertheless, he’s sure that Sofia would never resort to such criminal acts as this kidnapping. At least, he hopes not. He and the Doctor arrive at the TARDIS, and the Doctor sets the console to scan for phone calls -- and as he’d hoped, the kidnappers make a second call. He is unable to trace its source, but does trace its destination, and sets off in the hope that the kidnapper was calling the ringleader behind the operation. The TARDIS materialises in a set of underground tunnels, about 30 metres beneath the city, and the Doctor and Rossiter set off to investigate, unaware that they’re being watched.

Sofia tries to set off on her own in the blade shuttle, but Jenner refuses to let her endanger her life. When he threatens to restrain her by force unless she explains where she’s going, she gives in and admits that Sebastian has been acting strangely lately. She doesn’t want to believe that he’s involved in the kidnapping, but he isn’t answering calls at his flat, and she has thus decided to look for him at his old penthouse, which his parents bought for him and where they used to spend time together in medical school. Jenner insists upon accompanying her, and despite her protests, he does so armed; she may claim that Sebastian isn’t a violent man, but Jenner has a bullet wound in the arm that says different.

The Doctor and Rossiter follow the sound of unearthly moans to a room that looks like a hospital ward. The patients seem docile at first, but when the Doctor breaks open the electronically sealed doors, the patients grow aggressive and attack. The Doctor and Rossiter retreat into a small office and barricade the door, and as the crazed patients try to break through to get to them, the Doctor starts sorting through the files in the office, looking for some clue as to what’s going on. Again, he suggests that Sofia may be connected to these events, and the exhausted Rossiter snaps at him, telling him that it’s not Sofia’s behaviour that’s got Evelyn so worked up. He apologises, but the Doctor understands what Rossiter is trying not to tell him -- and Rossiter admits that Evelyn had always hoped that the Doctor would show up to give her away at her wedding. The Doctor finally accepts just how childish he’s been, and hopes that Evelyn can forgive him for it. In the meantime, however, he has more pressing concerns. The office contains files on ordinary out-patients -- and coma patients with no family or friends, whose life-support systems were turned off by authority of the hospital administration. To Rossiter’s horror, Evelyn is one of the out-patients whose file is stored here; one of the “dead” coma patients is attacking the office at this very moment; and the death certificates were signed by Sofia.

As Sofia tends to the grievously injured Sebastian, Mel notes that Sofia’s showing more concern for the kidnapper than for his victim; however, Sofia furiously points out that Sebastian has been shot six times at point-blank range, while Evelyn just has a headache. Evelyn starts to convulse while they are being transported to hospital, however, and Sofia realises that her condition is more serious than she’d thought. She thus contacts Dr Szabo, who was dealing with other matters in his office; nevertheless, he takes the call when he learns it’s from her, and agrees to arrange for both Sebastian and Evelyn to receive immediate treatment. Mel notes that Sofia seems to regard Dr Szabo almost as a figure, and acknowledges that she’s been rather hard on her; however, she decides to reserve judgement, especially as even Jenner can’t be sure that Sofia didn’t deliberately manipulate Sebastian into attempting to kidnap Evelyn. Meanwhile, the unconscious Evelyn dreams of her disappointment when the Doctor failed to show up for her wedding to Rossiter.

The shuttle arrives at the hospital, where Dr Szabo assures Sofia that her father would be proud of how she’s handled herself and promises to operate on Sebastian himself. As Sebastian is wheeled off to the operating theatre, he deliriously babbles about his failure to medicate the coma patients, but Mel pays little attention; she’s more concerned with Evelyn, who is taken away for a battery of tests. Jenner finds that he’s unable to contact the Doctor or Rossiter, and Mel deduces that they’re following another lead, perhaps to whomever Sebastian called from the penthouse. In any case, she decides that she owes Sofia an apology.

Szabo finishes work and assures Sofia that Sebastian will recover fully once his wounds are sutured. Mel then arrives to speak with Sofia, but the shaken Sofia refuses to speak to her, especially when Mel claims that Sebastian was working with someone else and casually suggests that Szabo might be a suspect. Sofia slaps Mel across the face and informs her that, after the Killoran invasion, Szabo tended to the invaders’ injuries as well as his own people’s -- an act of kindness that many of his fellow Világians regarded as an act of treason. It’s taken a long time for his reputation to recover, and Sofia regards him as a true hero. However, as she lashes out at Mel for ignorantly bad-mouthing her mentor, Sebastian goes into cardiac arrest, and Mel rushes off to find Dr Szabo as Sofia rushes into the OR to see what she can do.

Sofia reaches the OR too late to save Sebastian, and she remains to grieve even when she is called to Theatre 4 to attend Evelyn. However, Jenner then fetches her in person, telling her that Evelyn is beginning to convulse. Sofia pulls herself together and checks her stepmother’s cranial scan, and is shocked to see that a blood clot has formed in her head remarkably quickly; if they don’t operate immediately, Evelyn will die. Meanwhile, the unconscious Evelyn dreams of her marriage to Rossiter, and of how the Doctor failed to show up...

The Doctor and Rossiter free the abused Killorans from the medical torture devices they’ve been strapped into while the Doctor begins to work out what’s going on. Presumably, the Killoran DNA was administered to Evelyn during her recent operation, and although Dr Szabo has always been principled and apolitical, Sebastian was Dr Szabo’s theatre assistant at the time. However, this doesn’t explain why Sofia signed the death certificates of the coma patients, patients who -- like Evelyn -- have presumably become uncontrollably aggressive due to the Killoran DNA in their systems. In any case, Evelyn is clearly in need of medical attention, and the Doctor may have no choice but to leave the traumatised Killoran prisoners behind while he finds her.

Mel rushes into Szabo’s office to tell him about Sebastian, only to find him watching the Doctor, Rossiter and the Killorans on a CCTV monitor. Szabo claims to have discovered that Sebastian was conducting unethical experiments in the abandoned tunnels beneath the hospital, which once belonged to the Galen royal family. He admits to killing Sebastian in the theatre, but claims that he did it to avenge those whom Sebastian had killed in the course of his experiments. Mel isn’t entirely convinced, but then realises that the Doctor is in trouble and rushes off to rescue him. Before following her, Szabo types a command into the keyboard -- and down in the tunnels, the doors to the medical ward click open, releasing the aggressive coma patients into the corridors. Unaware of what Szabo has done, Mel accompanies him to the cellars, where he claims to have found Sebastian’s secret lift. However, the lift opens up onto an empty shaft, and before Mel can react, Szabo pushes her in headfirst...

The Doctor and Rossiter are just prying open the lift doors on the level beneath, and the Doctor catches Mel as she falls. Shaken, Mel reveals that Szabo is the mastermind behind the experiments. The coma patients are approaching, and the Doctor realises that he and Rossiter will have to try to rescue the Killorans, who are in no fit state to defend themselves. Even after what they did to his people, Rossiter is ashamed by what one of his people has done to them. Mel finds that she can communicate with the Killorans, thanks to the TARDIS’ ability to translate for her, and she guides them to the lift shaft, where the Doctor has pried away the doors and is hoping that the Killorans can climb up to safety on the next level. It becomes clear, however, that the mob of blood-crazed patients will reach them first -- but at the last moment, Jenner arrives from a second lift and drives off the crazed patients at gunpoint, shooting only to wound. He tells Rossiter that Sofia is about to operate on Evelyn, but the Doctor realises that if Sofia is unaware that Evelyn has been given Killoran DNA, the shock of surgery could kill her. Ordering Jenner to stay with Mel and get the Killorans to safety, the Doctor and Rossiter rush off to the operating theatre. Meanwhile, Szabo returns to his office, sees what’s happening in the cellars, realises that events have moved on out of his control, and philosophically pours himself a drink.

Evelyn goes into cardiac arrest before Sofia can begin to operate, but Sofia stops the crash team from trying to revive her, realising that Evelyn’s heart fibrillation is a symptom not of her current condition but of her underlying heart defect. The Doctor bursts in, takes in the situation, and quickly explains that Evelyn has been given a transfusion of Killoran DNA that is overwhelming her human system; however, if he injects a sample of Evelyn’s blood into himself, his Gallifreyan immune system will quickly manufacture antibodies for the foreign material, which Sofia can then inject into Evelyn. While the shock of surgery would have killed her, the infusion of antibodies teachers her system how to deal with the problem on its own; soon, the blood clot will dissolve, and Evelyn’s rage and headaches will stop.

The Doctor sets off to confront Szabo, along with Mel, who has left Jenner and the hospital staff to deal with the Killorans and the aggressive patients. Szabo is sitting in his office, drinking and watching events unfold on his monitors, and Sofia arrives just in time to hear him confess that he’s been manipulating her all along. By acting the role of a father figure, he tricked her into fronting the campaign against Killoran technology, thus distracting people from the possibility that unscrupulous scientists might wish to study the Killorans themselves. He freely admits that he hates the Killorans for what they did to his planet and feels no compunction about torturing them for it; however, he’s been practical about his revenge, and has taken the opportunity to experiment with their DNA and search for a way to speed up the healing process in his patients. The experiments haven’t been going well, as all who received the Killoran transfusion developed aggressive tendencies; the coma patients became mindlessly destructive, and the out-patients have been causing the increase in the city’s domestic violence statistics.

Szabo explains that Sebastian stumbled across his operation, but was impressed and offered to help; through him, Szabo got Sofia to sign the coma patients’ death certificates, although she was unaware of the truth. Unfortunately, Sebastian’s unrequited love for Sofia has led to his misguided attempt to kidnap Evelyn and thus to Szabo’s being exposed. Szabo tried to cover himself by killing Sebastian in the operating theatre and pushing Mel down the lift shaft, but once he saw Jenner in the tunnels, he realised that Sofia’s bodyguard was more than he seemed and that higher powers had had him under investigation for some time. Now that he knows this, he’s taken the only way out. Too late, the Doctor and Mel realise that Szabo has poisoned himself with his last drink, and they can do nothing as he keels over at his desk. Sofia controls herself, realising that it’s now up to her to put things right -- and the Doctor and Mel compassionately offer to help.

Meanwhile, Evelyn has an unexpected visitor in the recovery room: the Seventh Doctor, who has come to lay some ghosts to rest. The death of Cassie has always been between the Sixth Doctor and Evelyn ever since they escaped from the Forge, and it’s the one thing that the Doctor was never able to put right. But now the Seventh Doctor shows Evelyn a photograph of his new travelling companion, Thomas Hector Schofield. He’s a bright young man, cheerful and loyal... and everything that Evelyn would have expected of Cassie’s son Tommy. Unlikely as it seems, the Doctor just happened to stumble across him, and “Hex” chose of his own accord to start travelling with him, unaware of the Doctor’s connection to his past. The Seventh Doctor bids Evelyn au revoir and departs, leaving her finally at peace, knowing that some good has come of the tragedy that she was forced to witness.