The Doctor, Tegan and Turlough are visiting the Eye of Orion, a tranquil place. As they are enjoying their down time, the Doctor begins to feel cosmic angst. He explains he's being diminished, whittled away, piece by piece. Urgently he rushes to set the co-ordinates of the Tardis, trying to be reunited with his other selves. As he sets the Tardis in motion, he passes out on the floor and begins to disappear. Tegan and Turlough are uncertain how to help the Doctor or what he's babbling about, so resort to arguing amongst themselves.
Meanwhile on Gallifrey, a sinister Timelord is using ancient and forbidden equipment to collect previous versions of the Doctor, his companions and a few of his enemies and depositing them in Gallifrey's 'Death Zone'; a place where in ancient days, the Timelords would kidnap aliens from all over the universe and have them fight against each other. Using the 'Time Scoop', he collects the first Doctor walking alone in a garden, his grand-daughter Susan, The Second Doctor as he's visiting the Brigadier at his retirement from Unit, the Third Doctor as he's driving Bessie and Sarah Jane Smith on her way to work. He tries to collect the Fourth Doctor and Romana but the equipment suddenly fails.
As the first Doctor is wandering in a corridor the Time Scoop placed him, he runs into his grand-daughter. Their reunion is short lived as they meet a Dalek which begins chasing them. The Doctor tricks the Dalek into shooting itself. The Dalek explodes, taking portions of the wall with it. Just outside, the Doctor and Susan see the Tomb of Rassilon and realize they are on Gallifrey. They make their way for the 'Dark Tower' when Susan spots the Tardis.
The Second Doctor and the Brigadier are walking along trying to figure out where they are when they have a brief encounter with a Cybermen. Suddenly the Doctor realizes they could be playing the game of Rassilon when he spots the Dark Tower. He explains to the Brigadier how at one point in Gallifrey's history, they used to kidnap others and bring them to fight in the games till other Timelords rebelled and forbade the use of the equipment. The Brigadier can tell by his tone, they are not expected to win.
Realizing what's going on, the High Council of Gallifrey (minus two who went into the Death Zone to investigate, neither returned) look for the Doctor to help figure out who's using the ancient equipment. When they can't find the Doctor they employ the Master to rescue the Doctor and help figure out who's using the equipment. President Borusa objects but is over ruled by the High Council. The Master also is reluctant, but agrees; finding humor in being sent to 'rescue' the Doctor. He is given a recall device which will return him back to the High Council chamber. He's also given the 'Seal of Rassilon' to help prove to the Doctor his good intentions.
The Third Doctor is driving Bessie when he hears a scream. Sarah, wandering around blindly in the fog has fallen down a hill. Using Bessie and a rope, he rescues her, but she's surprised because from her perspective, the Doctor has already regenerated. As they drive away he tells her about the Death Zone when they run into the Master. Being his greatest enemy, the Doctor refuses to believe the Master is there to help. The seal of Rassilon does little to impress the Doctor. He assumes it's either forged or stolen and takes it away from the Master. Suddenly a strange energy bolt is fired and the Doctor speeds away, abandoning the Master, assuming its a trap. What the Doctor doesn't realize is the Master is being fired upon also. Suddenly, Bessie is hit and the Doctor and Sarah must start walking towards the Tomb of Rassilon.
Upon entering the Tardis, the First Doctor and Susan are surprised to find three young people in his Tardis. Tegan explains it's the Fifth Doctors Tardis. Suddenly he regains consciousness and life is restored to him. Susan is pleased to meet another, younger version of her grand-father, but Tegan realizes the two Doctors should not be together at the same time. The Tardis has been locked by a tractor beam so they set the co-ordinates for the Dark Tower. The Fifth Doctor, Tegan and Susan head for the Dark Tower to determine what is going on, who might be behind it and to free the Tardis.
As the Fifth Doctor, Susan and Tegan are walking, the meet up with the Master. The Doctor advises Susan and Tegan to back away as he talks. The Master tries to convince the Doctor of his good intentions and explains he is armed and could easily kill him if he wanted to, but the Doctor knows that isn't the Masters style at all. Suddenly a troop of Cybermen show up. Susan and Tegan head for the Tardis while the Master and the Doctor run in the opposite direction, followed by the Cybermen. When the Master is grazed and the Cybermen catch up to the Doctor he finds the Masters recall device and activates it. From a safe distance, Tegan and Susan watch the Cybermen catch up to the Doctor, but see him suddenly disappear. They run back to the Tardis to inform the First Doctor, but Susan trips and sprains her ankle on the way.
Back on Gallifrey, the High Council are surprised to see the Doctor arrive. When they explain what has been going on, he realizes the Master was telling the truth. The Doctor finds the High Council are just as clueless as he is. However, suddenly the black scrolls of Rassilon are found in the Castellans room. He claims to have never seen them before and is adamant he is not the one running the Time Scoop. But when he's taken away for questioning, the Doctor hears laser fire and finds the Castellan dead. The guard explains he was armed and trying to escape.
The Third Doctor and Sarah are close to the Tomb of Rassilon when they run into a Raston Warrior Robot, the most perfect killing machine ever created. Sensors detect movement, any movement. They move like lightening and their armory is built in. The Raston begins to toy with them when it suddenly spots a more delicious target, a group of Cybermen. While the Raston Warrior decimates the troop, Sarah and the Doctor escape, stealing the robots extra weapons and rope.
After escaping the Raston Warrior and the Cybermen, the Third Doctor and Sarah have climbed to the top of a rock face opposite the Tomb of Rassilon. The Doctor uses the rope acquired from the Raston to lasso a pillar at the top of the Tomb. Using it, he and Sarah "Zip-Line" across seconds before a lone Cybermen catches up to them, but dies before firing a shot. Safely across, the two enter the Tomb and search for the main hall.
Back at the Tardis, Susan informs her grand-father what has transpired. Realizing it's now up to him, he heads for the Dark Tower. Tegan offers to accompany him while Turlough bravely opts to remain in the Tardis with the now injured Susan. As the Doctor and Tegan enter the Tower, they realize they have been followed by the Master. After the Fifth Doctor left him, he formed an 'alliance' with the Cybermen, though he still warns the Doctor to hide before they Cybermen arrive. As the Cybermen enter, the Master instructs them to march in and take control of the Tower. What the Cybermen don't know is the floor is booby-trapped and the Master succeeds in destroying the entire cyber troop. The Master flees, telling the Doctor they too can cross the board safely, "It's as easy as pi". Deducing what the Master meant, the Doctor uses a mathematical formula to safely cross the floor, Tegan follows.
As they arrive upon a cave, the Second Doctor begins reciting a nursery rhyme. He explains to the Brigadier the Gallifreyan nursery rhyme talks of three entrances into the Tomb of Rassilon. One above, one between, one below. The cave is the entrance to the 'below', but their troubles are far from over as they realize they are being followed. Finding a safe enclosure, they discover they are being tracked by a Yeti, left over from the former games. When the angered Yeti starts a rock slide, they believe they are trapped. But the Doctor discovers the cavern goes back further and they find a door leading into the Dark Tower.
Still in the Capitol, the Fifth Doctor does a bit more investigation work and realizes the Castellan was framed. Searching about, he discovers a hidden room containing the forbidden Death Zone equipment. The biggest shock however, is Borusa, the Lord President of Gallifrey is operating the equipment. His ambition is to remain the President of Gallifrey eternal. The Doctor vows to stop him, but Borusa has on the Coronet of Rassilon which emphasizes his will. He uses it to make the Doctor bow down and follow him.
According to legend, Rassilon can offer eternal life to whoever seeks it. This is the reason Borusa sent the Doctor into the zone, to eliminate all threats and prepare the way for him to arrive. When he does, the other Doctors concentrate and break Borusa's hold of the Fifth Doctor. But Borusa does not feel threatened, he plans to tell the High Council the Doctor is the traitor. Considering the Doctor is the notorious renegade, he assumes the Doctor will not be trusted.
His hold of the Doctor released, Borusa is still able to command all the companions to remain still, unmoving. Earlier the companions banded together to dispose of the Master who is now bound on the floor, so he's not a threat either. Things are certainly in Borusa's favor when suddenly the voice of Rassilon is heard. A holographic image of his head appears over his preserved body and offers immortality to whoever takes, and puts on the ring from his dead body. Gleefully, Borusa puts on the ring, only to realize too late his immortality comes at a price. He's turned into a living stone bust, adorning Rassilon's sarcophagus.
The First Doctor begins to laugh when he suddenly realizes what the old Gallifreyan quote, "To lose is to win, and he who wins shall lose" means. As losers, the Doctor has won, and as the winner of immortality, Borusa is actually the loser. As a result of temporal fission, the first three Doctors depart, accompanied by the Brigadier, Susan and Sarah Jane Smith. Rassilon also frees the Maser and the Fourth Doctor. He then congratulates the Doctor on choosing wisely. Rassilon fades away just as Chancellor Flavia (the last remaining High Council member) arrives with a few guards. The Doctor informs her of Borusa's failure. Proud of his accomplishments, the Chancellor informs the Doctor he is to become the next President of Gallifrey. The Doctor reluctantly agrees and quickly ushers Tegan and Turlough into the Tardis.
As he rushes into the Tardis, the Doctor informs the guards to escort her back to the Capitol as he follows in his Tardis. Inside, Tegan and Turlough are distraught. Tegan doesn't want to go and Turlough hopes his Timelord buddies can find a Tardis that works properly and get him home. Surprised, the Doctor tells them he has no plans to return to Gallifrey. Instead he plans to go on the run from his own people, in a rackety ol' Tardis; The way it all began.