The ruthless Bannermen have all but destroyed their enemies the Chimeron. The last of the Chimeron and their beautiful humanoid queen, Delta, try to escape on a Bannerman ship, but Gavrok kills the last Chimeron before he can escape. Delta escapes with a silver egg-shaped package, and Gavrok sets off in pursuit of her, the last Chimeron survivor.

The TARDIS materializes at a space tollport, where the Doctor and Mel emerge to find that they're the tollport's ten billionth customers and have thus won a trip to Disneyland in the year 1959. The alien Navarinos' 1950s Club have physically altered themselves to resemble humans, and are preparing to travel back in time on a space cruiser disguised as a 1950s bus; the Doctor, however, recognizes Nostalgia Trips as the most accident-prone tour company in the Universe and decides to follow on behind in the TARDIS. Just as the bus is about to take off, Delta arrives at the tollport and rushes on board, as Gavrok and his men try to compensate for her sudden change of direction.

The Americans have launched an experimental satellite, and CIA agents Hawk and Weismuller are in Wales with orders to monitor its progress. The satellite, however, collides with the bus while its driver, Murray, is talking with Mel and the reticent Delta, and the bus begins spiralling out of control towards Earth. The Doctor uses the TARDIS to generate an anti-gravity loop, gently lowering the bus to Earth near the Welsh holiday camp Shangri-La. The camp leader, Burton, assumes them to be the holiday group he was expecting to arrive, and when Murray accidentally breaks the replacement warp crystal he was trying to fit to the bus' engines, the Doctor advises the Navarinos to stay here for the night while he grows a new one.

Mel shares a room with Delta, but her attempts to get her roommate to open up are met with cold silence. Delta panics and pulls a gun on Mel when she hears the dinner gong, but reluctantly concedes that Mel doesn't seem to pose a danger and lets her go. Mel takes her story straight to the Doctor, who worries that Delta may not be the only one in danger if she's who he suspects she is. Meanwhile, Weismuller and Hawk receive new orders from America; they're to look for the downed satellite, which apparently landed somewhere in this area. Elsewhere, the Bannermen storm the tollport, question the terrified tollmaster, and kill him when he's unable to help them locate the missing tour bus.

That night, Delta attends the "getting to know you" dance in a dress she borrowed from Mel. Billy, the camp's mechanic, is singing at the dance with his band, using special amplifiers he built himself; he's fallen in love with Delta at first sight, much to the horror of his childhood friend Rachel ("Ray"), who's been nurturing an unrequited crush on Billy for years. Delta leaves the dance and returns to her room, where she apologizes to Mel for her behaviour and begins to tell her the truth... just as her silver egg-shaped package hatches into a green, wrinkled Chimeron baby.

Gavrok sends an energy pulse down the path of the homing signal, frying Keillor and incidentally knocking the Doctor and Ray unconscious. Meanwhile, Billy arrives to surprise Delta with flowers, and is surprised himself by the green little creature she's holding. She agrees to tell him and Mel the whole truth.

The next morning, Billy offers to take Delta to a beauty spot he knows of in the hills, while Mel, exhausted, falls asleep. The Doctor awakens just in time to see Billy and Delta driving off, unaware that Gavrok and the Bannermen are on their way. He and Ray awaken and warn Mel of the danger, and she leaves to warn Murray and the Navarinos while the Doctor and Ray try to convince Burton of the danger. He's unconvinced until the Doctor, in desperation, shows him the inside of the TARDIS; Burton then agrees to evacuate the staff from the camp while he remains to protect his livelihood. The Doctor hands the nearly-completed warp crystal to Murray, who desperately tries to will it to hurry while the Doctor and Ray set off to find and warn Billy and Delta.

The Doctor and Ray search the local beauty spots, and eventually track down Billy and Delta; Delta is feeding her child a special food which is causing the child to metamorphose into a beautiful Chimeron princess. The Princess sings in a high-pitched voice which can be either a song of peace or an attack frequency for use against the Bannermen. The Doctor and Ray arrive and warn Delta of the danger, and they set off back to the camp.

The Bannermen land in a nearby field, capture the puzzled Weismuller and Hawk, and set off in search of the camp they were told about. Murray has just installed the warp crystal and is preparing to take off when the Bannermen arrive and open fire, destroying the bus and all the Navarino tourists inside. Mel, horrified, tries to convince Gavrok that the Chimeron Queen died on the bus, but just at that moment, the Doctor and his friends arrive. They make a quick U-turn and flee, pursued by Bannermen, but as Gavrok prepares to kill Mel for her deceit Burton arrives and advises him to take her hostage instead, to use as bait.

Despite their reservations, the Doctor, Mel and Burton continue on, board the motorcycle and depart, while the Bannermen apparently do nothing. Gavrok fires a flare which attracts the attention of Arrex and Callon, the two Bannermen left to guard Weismuller and Hawk. Arrex and Callon bind the two CIA agents in a neck brace and head off to carry out their new orders, but as soon as they've gone Ray arrives and picks the lock of the neck brace. Arrex and Callon set themselves up on the nearby country lane and fire at the Doctor and his companions as they pass. They appear to miss, but the Doctor knows better, and pulls off the road some distance away to dispose of their tracking device. Meanwhile, Gavrok locates the TARDIS and places a booby-trap on its roof. He tests the trap by throwing a twig, which bursts into flames as soon as it touches the TARDIS.

Goronwy feeds his bees and proudly shows them and his collection of classic honeys off to Billy and Delta. The bees are producing royal jelly, the food which turns an ordinary drone into a queen bee; Billy, drawing the obvious analogy, steals a package of the food which Delta has been feeding the Princess. Arrex and Callon, searching for the missing CIA agents, stumble across Goronwy's home and see Delta and the Princess in the backyard; but just as they're about to open fire, the Princess screams -- a shrill, piercing alien sound which bores through the Bannermen's heads. Callon flees, clutching his ears in pain, as Delta shoots Arrex.

Realizing that Gavrok and his men will soon arrive, the Doctor turns on the radio, plants Delta's scarf in the honey shed, and leads the others back to Shangri-La. Gavrok and his men follow the tracking signal to a nearby field only to find that the Doctor has tied the tracer to a goat; Callon then arrives with news of the Chimeron's location, and the Bannermen rush to Goronwy's and storm the house, hearing the radio playing inside. They eventually find Delta's scarf and break into the shed, but all of Goronwy's classic honey drops from the shelves and spatters over the Bannermen. They emerge, dripping with honey, and are immediately attacked by angry bees.

Back at Shangri-La, the Doctor notices the charred twig near the TARDIS and realizes what Gavrok has done. Meanwhile, Delta finds Billy consuming the "royal jelly", hoping to turn himself into a Chimeron so he can remain with her. The Bannermen return to Shangri-La, and the Doctor and Billy quickly set up their contingency plan; Billy wires his band's amplifiers to the camp's PA system, and as the Bannermen move into position for an assault, the Princess screams into the intercom. Her shrill cry sends the Bannermen into spasms -- and Gavrok, studying the TARDIS to find out why nobody stepped into his trap, clutches his ears in pain, stumbles into his own trap and is instantly disintegrated.