An experimentation with form, "Flip-Flop" was released as two discs (two episodes per disc) that could be listened to in either order. The story is cleverly constructed so that it makes sense either way.
Black DiscIt's Christmas Eve 3090. The Seventh Doctor and Mel turn up on the planet Punxatornee, where the human population exists as slaves to an alien slug race, the Slithergee. The local president, Bailey, has spent the past 30 years surrendering all human dignity to the Slithergee just to appease them. The Doctor and Mel don't plan to meddle, because they're right in the middle of a battle against some Quarks. They've only really popped over to Punxatornee in order to fetch some Applied Phlebotinum.They're promptly captured by a man named Potter of the planetary police, who were already looking for them, believing them to be dangerous terrorists. A pair of humans (Reed and Stewart) watch the arrest. Stewart notes to Reed that Mel was right about having a time machine. So they bust the well-known terrorists out of jail, after a local Mad Doctor, Professor Capra, tortures the Doctor and Mel for a bit using an elaborate torture machine (that happens to use the Applied Phlebotinum that the Doctor was looking for). The Doctor and Mel are grateful but confused — first the police, and now Stewart and Reed, claim to know them. President Bailey is confused, too — how could the Doctor and Mel have escaped, considering that Stewart and Reed had been shot to death not an hour before?Stewart explains that he and Reed are the last surviving members of La Résistance. Since the Doctor conveniently has a time machine, Stewart forces him (at gunpoint) to take them back 30 years into the past and prevent President Bailey from surrendering the planet to the Slithergee. They arrive just in time to find Bailey in a state of undress with her secretary, cheating on her secret boyfriend, Deputy Mitchell. Stewart kills both Bailey and her secretary, hoping to blame her death on the latter. Mitchell is left behind to become president and, mad with grief, declares war on the Slithergee.The Doctor wearily takes everyone back to the future, where Stewart and Reed are horrified to find out that the planet has turned into a radioactive wasteland. Twenty years before, the planet was devastated by an attack from the Slithergee, who had been refused permission to settle there. As they rush into the city, hoping to find some proof that everything's going to be fine, the Doctor is forced to follow them when he realises that Stewart stole his Applied Phlebotinum (which the gang picked up over the course of the plot).Back inside the TARDIS, Stewart forces the Doctor to take everyone back to just before he took everyone back the first time (even though the Doctor protests that that timeline doesn't exist anymore). The Doctor does so, and while Stewart and Reed rush off against the Doctor's advice, the Doctor and Mel are confronted by officer Potter, the same man who "recaptured" them when they arrived in this history — but from Potter’s perspective, this is the first time he’s ever seen them. The Doctor therefore claims that he and Mel are Slithergee agents, and the bewildered Potter arrests them and takes them back to the central bureau.Meanwhile, the Stewart and Reed that grew up in this timeline are lieutenants in the colony security forces. The Doctor figures that he has to be captured, since he was said to have escaped when they first arrived. Their quick escape (by decrypting the cell’s cipher lock) prompts the professor Capra of this timeline, who doesn't understand how anyone can know about his "machine", to issue a warrant for the arrest of the Doctor and Mel. Lieutenants Stewart and Reed are sent out to re-capture them. The Stewart and Reed from the Slithergee timeline quickly follow after them.The Doctor realises that his self from this timeline is about to materialise his TARDIS in the exact same spot where the TARDIS is now, when he'll first arrive on Punxatornee. Since that would cause a paradox that could shred the Web of Time to pieces, he vworps out just in time to go deal with the Quarks, leaving the Doctor and Mel of this world to deal with the planet.
White DiscIt's Christmas Eve 3090. The Seventh Doctor and Mel turn up on a radioactive wasteland that was once the planet Punxatornee. Twenty years before, the planet was devastated by an attack from the Slithergee, who had been refused permission to settle there. The Doctor and Mel don't plan to meddle, because they're right in the middle of a battle against some Quarks. They've only really popped over to Punxatornee in order to fetch some Applied Phlebotinum.Two humans, lieutenants Stewart and Reed, recognise the Doctor and Mel as soon as they leave the TARDIS and immediately arrest them. They identify the Doctor and Mel by name and claim that they’ve already confessed to being Slithergee agents and knowing of "the Professor’s machine". The Doctor is unable to decrypt his cell’s cipher lock; according to Stewart and Reed, he’s escaped that way once already. However, they get rescued by other versions of Stewart and Reed (impersonating the lieutenants), who rather quickly get killed on the way back to the TARDIS.Meanwhile, professor Capra has spent years forced to perfect a time machine for Mitchell, the planet's president. Mitchell plans to use the machine to go back in time and prevent the assassination of his secret girlfriend president Bailey by her secretary (and secret lover — Bailey was two-timing), 30 years ago. The Doctor realises that Capra's machine uses the Applied Phlebotinum he'll need to defeat the Quarks, but a confrontation in the lab results in both Mel and lieutenant Stewart being accidentally sent back 30 years into the past. Reed and the Doctor follow them in the TARDIS, but they're unable to prevent Stewart from murdering Bailey's secretary, and the (apparent) assassination attempt is prevented. The Doctor tries to force himself to shoot Bailey anyway, in order to preserve the Web of Time, but can't bring himself to go through with it. When Bailey learns from them what her planet would become if she were to refuse the Slithergee her hospitality, she solemnly vows to appease them as much as she can, in order to save humanity.The Doctor wearily takes everyone back to the future, where Stewart and Reed are horrified to find out that the planet has turned into a slave colony for the now dominant Slithergee. As they rush into the city, hoping to find some proof that everything's going to be fine, the Doctor is forced to follow them when he realises that Stewart still has his Applied Phlebotinum (which he'd given to Stewart for safekeeping).As the Doctor and Mel search for Stewart and Reed, they hear the security alert identifying them as enemy agents, and are then caught by this timeline's officer Potter. When he calls Lieutenant Stewart, however, the lieutenant claims that he and Reed are still on the way to collect their prisoners. Suspecting that Potter is in league with the enemy agents, Mitchell orders the lieutenants to return to Capra’s laboratory; he then contacts Capra himself, who seems somewhat distracted as Mitchell warns him to lock the door and keep an eye out for the Doctor and Mel.Meanwhile, the Stewart from the radiation timeline orders the Doctor to take him and Reed back to the previous day so they can prevent themselves from going back in time. The Doctor and Mel try to point out that this is impossible, as the “yesterday” they came from no longer exists, but Stewart forces him. However, they’re forced to hide from Slithergee patrol 14 and Potter, the same man whom they saw killed when they arrived in this history. Stewart and Reed then arrive, demanding to know why the Doctor and Mel are breaking curfew — and when Mel greets Stewart and Reed by name, they are bewildered, as this is the first time they’ve ever seen her. The Doctor realises that these are different versions of Stewart and Reed who grew up in this version of history, and this version of Stewart is very interested in Mel’s claim to have a time machine. When the Stewart and Reed from the radiation timeline are confronted by Potter, Stewart remembers — too late — that when they first arrived in this history, they met and killed Potter, who claimed to have killed them already. Potter shoots them both.The Doctor realises that his self from this timeline is about to materialise his TARDIS in the exact same spot where the TARDIS is now, when he'll first arrive on Punxatornee. Since that would cause a paradox that could shred the Web of Time to pieces, he vworps out just in time to go deal with the Quarks, leaving the Doctor and Mel of this world to deal with the planet...