Iris calls a peace conference to announce her retirement, and in passing, she casts the press into a kerfuffle by wishing her successor the best of luck whoever she -- or he -- might be...

Panda awakens aboard the bus to find that Iris has saved his life, as well as the entire Universe, by baking an antidote to the Huldran plague in a cheesecake made from the milk of a giant bat. Unfortunately, there was only enough cheesecake for one -- and to Panda's horror, a man descends from the upper deck of the bus and introduces himself as the new Iris Hilary Wildthyme. The new Iris is a New Age self-help guru, and to Panda's even greater horror, he begins his new incarnation by pouring out the old Iris's entire stock of booze, including a bottle that Panda claims contains the soul of Noel Edmonds. Panda must physically attack the new Iris to reclaim a hip flask that he claims has great sentimental value, and Iris gives in on this one count.

Understanding that Panda needs time to deal with this transformation, Iris offers to take him to a party with the Emperor Caligula, but Panda spends the entire blow-out sulking and clutching his hip flask like a security blanket. The new Iris insists that he needs his old friend to teach him what kind of man he is, but Panda storms back to the bus, claiming that the real Iris would never have associated with a madman like Caligula, no matter how good the party was. Iris eventually returns to the bus himself to find Panda waiting impatiently; a phone that neither of them knew existed rang while Iris was out, and Panda has taken a message from a chap called Roger, calling Iris to Málaga in the space year 2109. The bus materialises in an alley behind a run-down retro dance club, and when Iris and Panda emerge, they are confronted by a Naxian. Iris flees, carrying Panda with him, but they are captured when Iris fails to take into account the Naxians' ability to sneak up on one.

Panda continues to clutch his hip flask as the Naxians toss him and Iris into a small back room in the club. As Iris frets that he has no escape plan, the club is rocked by explosions from outside; the city is being bombarded by xenon warheads. A Naxian warlord then enters the room and identifies himself as Roger, and breaks down crying when Iris doesn't recognise him. To Iris's humiliation and Panda's amusement, Roger reveals that he and Iris have been having an affair ever since the Radio Yesterday incident, when Roger was merely Third Naxian on the Left; ever since then, whenever Iris has fought the Naxians, she and Roger have nipped off in the bus in the middle of the adventure for a little fun on their own.

Roger explains that his team was sent back in time to establish a beachhead in 22nd-century Earth, but when Iris stopped returning his calls, he became despondent, gave up interstellar conquest, and opened a nightclub instead. However, the club has attracted unwanted attention from the North and South power blocs that control the world. Both blocs fear that the power of cheap music could unite the world, and they have thus banned all music but their own official national anthems. Málaga is technically an independent state, but both sides are bombarding the club nonetheless to prevent their citizens from being corrupted by its music. The club is protected by an electro-shield, but it's diverting the missiles onto the rest of the city, and the epsilon rays generated by the shield are cancelling out the sound waves from the club's speakers, preventing the club from playing music. Only Iris has the know-how to fix the problem, get the club's groove back and spread the love to the world again.

Iris agrees to help, and Roger supplies Panda with mojitos while Iris fixes the speakers; however, Iris privately admits to Panda that Roger is no longer his type, and that he intends to make a run for it once he's finished his work. The club fills with music again, and the delighted Roger lets slip to Panda that he deliberately put himself in danger so that Iris would be compelled to come save him. However, Iris then tries to make an excuse and return to the bus with Panda, and Roger realises that Iris is abandoning him once again. Furious, he merges with the other Naxian drones to form a huge gestalt, and attacks Iris and Panda. Iris begins to panic, and Panda finally gives up and opens the hip flask that he's been carrying around. Everything goes very quiet -- and to Iris's astonishment, the former Iris walks into the club and pours herself a drink.

Elsewhere, the Extreme Commander in Chief On Top of the North Bloc, aka Barry, receives distressing news. Not only is the annoying nightclub in Málaga still intact, it's begun broadcasting again -- and by sheer coincidence, the music is being broadcast at the same frequency used to activate the South Bloc's automatic weapons systems. The North Bloc's own automatic defence systems react immediately to the provocation, and there is no manual override. Within minutes, both sides have launched their entire nuclear arsenals at each other, which means that the world will be destroyed in just over 10 minutes.

In Málaga, the female Iris ushers the male Iris and Panda aboard her own bus. Panda explains to the bewildered Iris-2 that Iris did not regenerate, but took a holiday to the Rehab Dimension to recuperate from recent difficult adventures. Aware that she wouldn't be able to tolerate the company of washed-up rock stars and celebrity wannabes for very long of her own accord, she gave Panda a temporal link in the form of the hip flask, so that he would be in charge of deciding when it was appropriate to summon her back. Iris now leaves the bus to put Roger in his place; she never pretended that their affair was anything other than a bit of casual fun, and he should be ashamed of himself for threatening her friends and inciting an international incident over it. However, she promises to go out to dinner with him if he stops this at once.

The embarrassed Roger allows his drones to separate from him. As the drones shuffle off to give Roger and Iris some alone time, Iris-2 emerges from the bus, awed at how quickly she dealt with the dangerous situation. However, Panda then reports that the two power blocs have launched their nuclear arsenals. Iris uses the bus's unexpected phone to call up Barry and berate him for what he's done, but the enraged Barry blames the launch on the disco and reveals that he is unable to abort the missiles. Iris hangs up on him, intending to sort out the situation herself, and Barry angrily orders his subordinate to prepare a zipper-ship so that he can travel to the disco of corruption and deal with its owners in person.

Iris sends Panda and Iris-2 back to their own bus, which is still in the alley where they parked it, while Roger boards Iris's bus with her. He is still unable to understand how the two Irises can exist in the same place and time, and Iris explains that Iris-2 is not really her future incarnation; he and his bus are computer-generated avatars created by Iris's bus, to act as decoys while she was away. Too late, she realises that she's left the intercom on and that Iris-2 has heard everything. Stunned by this revelation, he nevertheless pilots his bus into orbit around the Earth, above the 1.3 million nuclear missiles hurtling through the atmosphere. On Iris's instructions, he then follows Iris's bus around the Earth, and the two buses travel so fast that they catch up with their own temporal exhaust. The globe is encircled with a fog of spatiotemporal distortion that confuses the missiles' guidance systems -- but rather than shutting down, the missiles lock onto the source of the distortion and begin to pursue the buses.

Iris and Iris-2 set course for the sun, intending to drag the missiles into its gravity well and then dematerialise. Unfortunately, the plan hits a snag when they realise that the missiles are fitted with temporal nose cones, which means that the missiles will still be able to pursue the buses even after they dematerialise. Since they appear to be doomed, Iris-2 asks Iris why she created him, and she explains that she needed someone to take her place in the multiverse so that her enemies wouldn't take advantage of her absence. The decoy concludes that he's served his purpose and will never be able to live up to the real Iris, and volunteers to make a narratively apt heroic sacrifice by driving his bus into the sun if Iris can figure out how to rescue Panda. But Roger protests that he prefers the decoy to the real Iris, who gads about without thought for the consequences or for how much she hurts other people without realising it. Panda also admits that he was upset with the real Iris for abandoning him, and that he took it out on the decoy because he'd been expecting Iris's replacement to be a younger and sexier female version.

The decoy admits that he doesn't really want to die, it's just that he doesn't know what to do with his life now. Nevertheless, Iris insists that the decoy has a life of his own, and reveals that Hilary isn't her real middle name; that's a name that the decoy imagined himself. Iris promises to sort things out for everybody, but before she can do so, Hilary's bus blows a time gasket, and all 1.3 million missiles catch up to it and explode. Iris pilots her own bus back in time and materialises inside the decoy bus, and once Hilary and Panda are safe inside, she pilots the buses into a temporal orbit around the moment of the explosion. Unfortunately, as long as the two buses are stuck together, they're unable to move away from this moment without getting caught in the explosion. It seems that someone will have to sacrifice themselves after all in order to get the decoy bus moving so that Iris's can escape in the other direction.

Hilary can now remember choosing that name for himself as the bus created him, and realises that he does have a life of his own after all. He thus points out that, rather than someone staying behind, they can all stay in Iris's bus and turn the decoy's ignition with a piece of string. Naxians weave their own nests out of sticky dribble, and Roger thus spits out a strand of mucous that they attach to the decoy bus's ignition. Hilary pulls the "string" at the same moment that Iris releases her bus's handbrake -- but the ignition doesn't catch. With seconds to go before Iris's bus is torn apart by the strain, Hilary leaps out to turn the ignition by hand, and Roger leaps out after him. Iris and Panda fly off helplessly to safety while the other bus explodes behind them. Iris touches down back in Málaga, and she and Panda mourn the loss of Hilary, whose emotional matrix turned out to be far more complex and realistic than they had expected. But Panda then notices that the bus's handrail is sticky, and he and Iris see that Hilary and Roger have attached themselves to the exterior of the bus using Roger's sticky dribble.