The Doctor is taking Erimem and her cat Antranak on a tour of their new home, now complete with evidence that Antranak has not learned to use the litter box. In the wardrobe, Erimem finds the outfits which the Doctor’s other selves used to wear, as well as clothing which belonged to his previous companions. Erimem realises that the Doctor has travelled with many people over the years, and that he’s older than he looks, but she’s always felt that there’s more to him than mere appearance. Touched, the Doctor resumes the tour, showing her the TARDIS cricket pitch and then opening up a door to the cloister room -- only to find a rainforest beyond. It seems that the TARDIS has been moving rooms around again on her own -- but although the Doctor doesn’t know it yet, he’s being observed, and the TARDIS is no longer under his control...
The Doctor begins to realise something’s seriously wrong when the door to his workshop opens up onto an army of terra cotta Chinese warriors he’d already delivered to the Emperor several regenerations ago. And the next door leads to the wardrobe, which they left three corridors back. The Doctor is upset by these inexplicable changes, and Erimem realises that he regards the TARDIS as his home. He admits that his home world will always be part of him, but that he no longer belongs there; he sometimes wishes he could settle down somewhere, but there’s just so much left of the Universe left to explore... and so much evil to fight. Erimem shares the sentiments, and admits that she’s sorry she never bid her mother farewell -- but her mother will now believe Erimem to be amongst the gods, and Erimem does not wish to disillusion her.
The Doctor tries to resume the tour, but the rooms shift about even more inexplicably -- and as he and Erimem find themselves back in the wardrobe, the TARDIS suffers a sudden power failure. A rack of clothes falls over on Erimem as the Doctor fetches out his everlasting matches, and he and Erimem realise that the shadows are moving strangely, as if there’s something else in the room with them. Hoping that Peri is safe in her own room, the Doctor and Erimem set off to search for the console room, but the temperature begins to drop rapidly and Erimem experiences ice for the first time in her life as condensation forms and freezes on the walls and floor. The Doctor ushers her on, unaware that they’re heading towards their deaths -- but the creature controlling the TARDIS and anticipating their destruction is thwarted when his surveillance systems suddenly cut out...
As the Doctor approaches another door, a voice speaks from the shadows, warning him not to open it -- and the door becomes transparent, revealing that it opens directly onto the Vortex. If the Doctor had opened the door, he and Erimem would have aged centuries in an instant. Their saviour steps from the shadows, identifying himself as Shayde, a corporeal manifestation of the will of the Time Lords used as their agent outside the Matrix. Shayde reminds the Doctor of the many times he has allowed his ship to be breached and damaged in this incarnation; on one of those occasions, the TARDIS was invaded by a life form which, if left unchecked, will use the Doctor’s TARDIS to conquer all of time and space. Only Shayde can navigate the shifting corridors of the TARDIS now, and he orders the Doctor and Erimem to stay where they are while he deals with the monster. However, the Doctor insists upon taking responsibility for his own mistake, and sets off with Erimem to put things right.
One very slippery journey later, the Doctor and Erimem find their way to the ship’s dimensional induction chamber, where the Doctor feels sure he will find the creature which has invaded his TARDIS. Shayde is already here, but is trapped in one of the force fields which the creature has set up to defend itself. Erimem is the first to spot the intruder, sitting proudly in the middle of an intricate set of controls -- and the Doctor is taken aback to find that his TARDIS has been taken over by a rovie, the Gallifreyan equivalent of a mouse, albeit a rovie ten times its normal size with a brain pan 1000 times larger than normal.
The Doctor and Erimem both suffer a fit of the giggles upon realising that the terrible threat described by Shayde is a mouse, but the rovie takes this as a great insult. He explains that he was once an ordinary rovie who lived in fear of being stepped on or sucked up by cleaning machines, until one day, while the Doctor was visiting his home world, the rovie took shelter from a cleaning machine inside his TARDIS. Ever since then, the TARDIS has provided the rovie with food and water. But things went wrong when Shayde, acting on the orders of his masters, installed a remote operation module in the TARDIS so the Time Lords could direct the Doctor to trouble spots throughout time and space, trusting him to sort things out without realising he was doing the Time Lords’ work for them. Unfortunately, when Shayde activated the unit, there was a leak in the temporal relays and the rovie was exposed to the fallout. The TARDIS tried to protect it, and thus instead of ageing to dust, the rovie evolved millennia within moments.
After its forced evolution, the rovie accessed the TARDIS data banks, taught itself about art, language and mathematics -- and then set about taking control of its home away from the Doctor. It hates the Doctor for the risks he has taken with the TARDIS, the rovie’s home, and envies him the company he has on his travels while the rovie has been alone foe years, without friends or female companionship. Erimem sympathises with this aspect of the rovie’s story; she too was isolated from ordinary people when she became pharaoh, but this all changed when she met the Doctor and Peri. The rovie, however, is convinced that the Doctor would never treat him as an equal, and thus intends to pilot the TARDIS back to Gallifrey and unleash a blast of the energy which mutated him. The Time Lords will be destroyed, while the rovies within the capital will be force-evolved as he was, and use the Time Lords’ technology to conquer all time and space.
As the rovie gloats over its imminent triumph, Antranak tracks down its mistress, who tries to set him on the mouse-like rovie. The rovie is enraged when she refers to him as a mouse, and the Doctor, identifying his enemy’s weakness, taunts him with insults and banter, telling him that the big cheese of the Universe shouldn’t get so ratty. Erimem joins in, and the enraged rovie approaches them, spitting out dire threats -- and realises too late that it’s stepped out of its protective force field. Erimem smashes the controls it used to take over the TARDIS, and Shayde, released from his force field prison, absorbs the protesting rovie into his body. He will take it back to Gallifrey, where the Time Lords will educate it and help it to become a constructive citizen. The Doctor sourly suggests that they offer it a seat on the High Council with the other rats, and demands that Shayde remove the remote control unit from his TARDIS. As Shayde departs, power is restored, and the Doctor, Erimem and Antranak resume their tour.