The Fifth Doctor, Tegan and Turlough are taking a break on the Eye of Orion, one of the most tranquil spots in the universe, when the Fifth Doctor suddenly collapses. Tegan and Turlough bring the Fifth Doctor back into the TARDIS, where they discover to their distress that he is literally fading away. The Fifth Doctor manages to set the TARDIS controls for a destination and the ship dematerializes.
In a hidden chamber, a dark figure is manipulating the controls of a time scoop and kidnapping the Doctor's previous incarnations out of the time stream along with some of his former companions. The First Doctor is taken while he is walking in a rose garden, the Second Doctor and Brigadier Lethbridge-Stewart from a UNIT reunion and the Third Doctor while he is out driving his roadster, Bessie. Also taken out of time are Sarah Jane Smith and the Doctor's granddaughter Susan Foreman. The Fourth Doctor and Romana are taken while punting along the River Cam, but whoever is doing this is frustrated as the two are trapped in the time vortex by a time eddy and unable to rematerialize. All of them, save the Fourth Doctor and Romana, are deposited on a desolate, rocky landscape — the Death Zone on Gallifrey.
Meanwhile, in the Capital on Gallifrey, the High Council of Time Lords, headed by Lord President Borusa and consisting of Chancellor Flavia and the Castellan, watches in concern. The Eye of Harmony is being drained by whoever is taking the Doctor out of time, endangering all of Gallifrey. Despite Borusa's misgivings, the High Council has unanimously voted to call in the Master to assist by going into the Death Zone to help the Doctors. Offered a pardon and a new cycle of regenerations, the Master accepts, and is given a copy of the Seal of the High Council by the Castellan to prove his bona fides, and a matter transmitter (transmat) recall device. He is then teleported via transmat to the Death Zone.
In the Zone, the Doctors face various dangers. The First Doctor and Susan are pursued by a Dalek through a hall of mirrors, finally escaping when they push the Dalek into a dead end, where the discharge of its energy weapon ricochets back and destroys it. The Second Doctor and the Brigadier escape from a squad of Cybermen, and the Third Doctor rescues Sarah from her fall down an embankment. Sarah is mildly confused, as she had seen the Third Doctor regenerate into the Fourth (Planet of the Spiders), but is glad to see the Doctor she once knew. The Second and Third Doctors explain to their companions that in Gallifrey's past, known as the Dark Time, the Time Lords misused their powers. A device called the Time Scoop was used to pluck beings out of their times and place them in the Death Zone, where they would fight each other in a sort of gladiatorial game. The Doctors' goal now is to reach the Dark Tower, where the Time Lord founder Rassilon is entombed, although there is some doubt as to whether Rassilon is actually dead.
The Master meets and tries unsuccessfully to convince the Third Doctor that he is there to help. He is then forced to flee when thunderbolts fall from the sky. The Third Doctor only sees this as confirmation that this is all a plot of the Master's. The First Doctor and Susan find the TARDIS and the presence of the First Doctor seems to stabilize the Fifth for the moment. Together, they scan the tower and find three entrances — one at the apex of the tower, the main gate at the base, and one underground, but a force field prevents the TARDIS's entry. The Fifth Doctor takes Tegan and Susan to go to the main gate, but encounters the Master, who has no better luck convincing the Fifth Doctor than he did the Third. At that moment, the two are surrounded by Cybermen, and when they try to run away, the Master is knocked out by a cybergun blast. The Fifth Doctor finds the Master's recall device on his unconscious body, and transmats himself to the Capitol. The Master, confronted by the Cybermen, offers himself as a guide to the Tower.
In the Capitol, the Doctor is informed of the situation by the High Council. The Doctor realizes not only that he has done the Master an injustice, but also that they were found too easily by the Cybermen. He opens the recall device and finds a homing beacon inside. The Castellan, who gave the Master the device, is arrested and his quarters ordered to be searched. There is found a box containing the Black Scrolls of Rassilon — forbidden knowledge from the Dark Time. Borusa destroys the scrolls before anyone can examine them and orders the Castellan taken to the mind probe for interrogation. However, as the Castellan is escorted outside, there is a shot. The Doctor rushes out to find the Castellan dead, and the Captain of the guard reporting that he was shot while trying to escape. The Doctor voices his concerns to Chancellor Flavia: the Castellan was stubborn, but not a traitor. There is more to this than meets the eye.
The Second Doctor and the Brigadier are exploring a series of caves when they encounter a Yeti left over from the games. Taking refuge in an alcove, the Doctor tries to chase the Yeti off with a firework, succeeding only in maddening it, and causing it to collapse the entrance to the alcove. However, the Doctor detects a breeze from further back and discovers the underground entrance to the Tower.
On the surface, the Third Doctor and Sarah come across a Raston Warrior Robot, according to the Doctor the most perfect killing machine ever devised. Able to move with blinding speed and fire bolts of metal at its targets, it detects its victims by motion. The Doctor and Sarah are unable to move without attracting the robot's attention, but luck is on their side when a squad of Cybermen come over the ridge and are rapidly eliminated by the robot. Taking advantage of the distraction, the Doctor and Sarah run past the robot's position, taking some rope and spare bolts from the robot's cave. Reaching a cliff face just above the Tower, the Doctor uses the rope and bolts to form a grappling hook, and he and Sarah abseil across to the top of the Tower.
Tegan and Susan have told the First Doctor what happened to the Fifth Doctor. The First Doctor decides to head for the main gate himself, with Tegan insisting on accompanying him. Opening the main gate through the means of a keypad hidden under a bell, they find a chessboard floor pattern blocking their way. The First Doctor determines that the chessboard is a trap — electrical bolts will destroy anyone attempting to cross unless they find the safe path. The Master appears at this point, warning them that the Cybermen are close behind. While the Doctor and Tegan hide, the Master lures the Cybermen onto the chessboard where they are killed. The Master tells the Doctor, "It's as easy as pie", then blithely steps across the board and moves into the Tower. The Doctor realizes that the Master means the Greek letter pi, and that the safe path is calculated by means of the mathematical constant. He and Tegan make their way across the trap. In the Zone, the TARDIS is being surrounded by Cybermen who start to assemble a bomb to blow it up. Inside, Turlough and Susan watch helplessly.
The Second and Third Doctors encounter more obstacles while moving separately through the Tower, with the mind of Rassilon exuding an intensifying feeling of fear. They also encounter what appear to be their previous companions: the Third meeting Captain Mike Yates and Liz Shaw; the Second meeting Jamie McCrimmon and Zoe Heriot. The Doctors soon realize that the 'companions' are just phantoms designed to impede their progress through the Tower, and the spectres vanish with a scream. Finally, all three Doctors reach the tomb where Rassilon's casket lies. While the Brigadier, Sarah, and Tegan get re-acquainted, the three Doctors try to translate an inscription written in Ancient Gallifreyan on a pedestal near a control panel.
The Fifth Doctor finds that Borusa has vanished from the Council chamber, but the guards insist that the President could not have gotten by them at the only entrance. The transmat is out of power, so the Doctor deduces that there must be a secret door. He finds it hidden behind a painting of Rassilon playing the harp. The key to opening the door is a series of notes played on the actual harp standing before the painting — notes indicated by the sheet music in the painting itself. The Doctor enters the secret chamber, finding the dark figure that had taken his other selves out of time: Borusa. The Lord President is not satisfied with ruling Gallifrey for his lifetimes — he wants to be President Eternal. Borusa has determined that Rassilon discovered the secret of immortality, and he means to claim it, sending the Doctors into the Zone to clear the way for him. Using the Coronet of Rassilon, Borusa overwhelms the Fifth Doctor's will, thus forcing the latter to obey his commands.
In the tomb, the Doctors have deciphered the inscription: Rassilon had discovered immortality and will share it with whomever overcomes the obstacles to the tomb and takes the ring from his body. However, one line troubles the First Doctor: "To lose is to win and he who wins shall lose." The Master steps out of the shadows to claim immortality for himself, yet is jumped from behind by the Brigadier and tied up by Sarah and Tegan. The Third Doctor fixes the control panel by reversing the polarity of the neutron flow, allowing the TARDIS to transport itself to the tomb (just seconds before the Cybermen's bomb detonates).
The Second Doctor contacts the Capitol and the Fifth Doctor answers, still under Borusa's control. He tells his other selves to await their arrival. He and Borusa transmat over to the tomb. Borusa paralyzes the Doctors' companions with a command and tries to control the minds of the Doctors as well, but fails as all four Doctors combine their wills against him. However, a booming voice echoes through the chamber — the voice of Rassilon, demanding to know who disturbs him. Borusa steps forward to claim immortality and while the other Doctors protest, the First Doctor holds the others back and says to the projection of Rassilon that Borusa deserves the prize. Borusa takes the ring from the body and puts it on. He finds himself paralyzed, then transformed into one of several stone faces carved into the side of the casket. Rassilon sends the Master back to his own time, then frees the Fourth Doctor from the time vortex and returns to eternal rest. The First Doctor smugly tells the Fifth that he finally understood the proverb. The 'prize' was yet another trap — a means for Rassilon to eliminate whoever sought immortality.
The Doctors and the companions say their good-byes to each other and re-enter the TARDIS save for the Fifth Doctor, Tegan, and Turlough. As those three watch, the others are transported back to their proper times. Chancellor Flavia arrives with guards and tells the Doctor that with Borusa's disappearance, the Council has appointed the Doctor as President. The Doctor appears reluctant, but Flavia tells him he cannot refuse an order of the Council or it will attract the severest penalties. The Doctor orders Flavia back to the Capitol, saying that he will travel there in his TARDIS and that she has full powers until his return. Once in the TARDIS, though, he reveals to Tegan and Turlough that he has no intention of returning. Tegan asks if the Doctor really intends to go on the run from his own people in a rickety old TARDIS. The Doctor replies, smiling, "Why not? After all, that's how it all started."
Doctor Peter Davison (Fifth Doctor)
Richard Hurndall (First Doctor)
Patrick Troughton (Second Doctor)
Jon Pertwee (Third Doctor)
Tom Baker (Fourth Doctor, previously untransmitted archive footage only)
William Hartnell (First Doctor, pre-titles archive footage clip only)
Companion Janet Fielding (Tegan Jovanka)
Mark Strickson (Vislor Turlough)
Carole Ann Ford (Susan Foreman)
Elisabeth Sladen (Sarah Jane Smith)
Lalla Ward (Romana II, previously untransmitted archive footage only)
Nicholas Courtney — The Brigadier
Anthony Ainley — The Master
Philip Latham — Lord President Borusa
Dinah Sheridan — Chancellor Flavia
Paul Jerricho — The Castellan
David Banks — Cyber Leader
Mark Hardy — Cyber Lieutenant
Richard Mathews — Rassilon
Frazer Hines — Jamie
Wendy Padbury — Zoe
David Savile — Colonel Crichton
Caroline John — Liz Shaw
Richard Franklin — Captain Yates
John Leeson — Voice of K9
Roy Skelton — Dalek Voice
John Scott Martin — Dalek Operator
Ray Float — Sergeant
Keith Hodiak — Raston Robot
Stuart Blake — Commander
Stephen Meredith — Technician
John Tallents — Guard
William Kenton — Cyber Scout
Writer Terrance Dicks
Terry Nation (The Dalek Invasion of Earth segment) (uncredited)
Douglas Adams (Shada segments) (uncredited)
Director Peter Moffatt
John Nathan-Turner (uncredited)
Richard Martin (The Dalek Invasion of Earth segment) (uncredited)
Pennant Roberts (Shada segments) (uncredited)
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6K
Series Season 20
Length 90 minutes
Originally broadcast November 23, 1983 (first global)
November 25, 1983 (first UK)