The Doctor, Tegan and Turlough have come to the peaceful Eye of Orion to rest and relax, but elsewhere, a black-clad Player is preparing for the first moves of his game... The Player's machinery locates the First Doctor wandering through a country garden and plucks him out of Time and Space. As the Player places the appropriate piece on his gaming board, the Fifth Doctor feels a sharp twinge of "cosmic angst" -- as if he'd lost something important. As more and more of his past is swept out of the timestream, he grows steadily worse, and staggers back to the TARDIS with Tegan and Turlough, desperate to locate his other selves...

The Second Doctor arrives at UNIT HQ for a reunion party, having read about the Brigadier's speech in the next day's newspaper. After successfully bewildering the new head of British UNIT, Colonel Crichton, the Doctor follows the Brigadier out into the grounds of UNIT HQ to reminisce over old times. But as he prepares to take his leave of his old friend, they are swept up by the Player's timescoop. The Third Doctor is taken while driving Bessie along a deserted country road; Sarah Jane Smith is captured while waiting for a bus, having ignored K9's warnings of imminent danger; and the Fourth Doctor and Romana are taken while punting along the river Cam. But things go seriously wrong -- the Fourth Doctor and Romana are caught up in a time eddy, and the Player is unable to retrieve them. The Fifth Doctor sets the TARDIS co-ordinates and dematerializes, but then faints -- and then his body begins to fade away. The Fourth Doctor's predicament is destabilising his very existence...

The High Council of Time Lords convenes to discuss a desperate situation, so desperate that despite President Borusa's disapproval, they have chosen to call on the Master for help. The ancient Death Zone at the heart of Gallifrey has somehow been reactivated and is drawing power away from the Eye of Harmony; worse, whoever is responsible has kidnapped the Doctor and placed his incarnations within the Zone. The Master is determined, ruthless, experienced and cunning -- and if he rescues the Doctor the Council has chosen to reward him with a new regenerative cycle. The Master is amused by the irony and accepts. The Castellan gives the Master a recall device with which he can transmat himself back to the Capitol once his mission is complete, and the Seal of Rassilon so he can prove his credentials to the undoubtedly suspicious Doctors. The Master is then transmatted into the Death Zone, where he finds the corpse of a Councillor who preceded him, and is pursued by lightning bolts from the sky...

The First Doctor is reunited with Susan in a maze of mirrors, but there is also a Dalek present in the maze. It pursues them, firing heedlessly after them, and the Doctor manages to outwit it by pushing it into a mirrored alcove; the Dalek fires blindly at them, and the bolts reflect from the mirrors and hit the Dalek itself. The resulting explosion blows out the wall of the maze to reveal that the Doctor and Susan are in the Death Zone on Gallifrey. Convinced that the answer to their dilemma lies in the Dark Tower at the heart of the Zone, they set off for the Tower only to find the TARDIS waiting for them in the wilderness outside. They enter to find Tegan and Turlough tending the unconscious Fifth Doctor, and the First Doctor's presence strengthens the Fifth Doctor, enabling him to recover and explain what he believes has happened. With his first incarnation's help, he sets up a trace to locate his other selves within the Zone, and then sets off for the Tower to investigate with Tegan and Susan.

The Second Doctor and the Brigadier evade a patrol of Cybermen near the remains of an old building, and the Doctor explains the Death Zone to the Brigadier; this is where the cruel ancient Time Lords abused their power by plucking creatures out of Time and Space and setting them to fight for the Time Lords' amusement. The Third Doctor is reunited with Sarah when she nearly falls over a cliff; he rescues her and explains the situation to her as well. As they approach the Tower, the Master finds and calls out to them, but the Doctor refuses to believe his good intentions and concludes that he's responsible for their presence here. Even when the Master produces the Seal of Rassilon the Doctor simply takes it from him, assuming that it's stolen. Before the Master can convince him otherwise more lightning bolts strike nearby, and the Master flees as Bessie takes a direct hit through her engine cowling. The Doctor and Sarah are forced to continue onwards on foot.

The Master then encounters the Fifth Doctor, Tegan and Susan, and the Doctor orders Tegan and Susan to wait while he speaks with the Master. He doesn't believe the Master's story either, and once again they are interrupted -- this time by a patrol of Cybermen. The Cybermen open fire when the Doctor and Master try to escape, and the Master is felled by a ricochet from an exploding stone. The Doctor finds and takes the Master's recall device and uses it to transmat himself to safety. Tegan and Susan flee back to the TARDIS to warn the First Doctor and Turlough, but on the way Susan sprains her ankle. She remains in the TARDIS with Turlough while the First Doctor decides to go to the Tower with Tegan. The Master recovers to find himself a prisoner of the Cybermen, and offers to lead them to the Tower, claiming that it's the citadel of the Time Lords who brought them here to fight as pawns in a game. The Cyber Leader decides to let the Master lead them to the Tower; he can be disposed of once his usefulness is over.

The Fifth Doctor is surprised to find himself back in the Capitol, and realizes that he did the Master an injustice. But so did somebody else. The Cybermen and Daleks were never used in the Games in the past, as they fought too well; somebody must have reactivated the Timescoop in order to bring the Doctor's old enemies to the Zone. And they were found far too easily -- which is explained when the Doctor pops open the Master's recall device to reveal a homing beacon nestled in amongst the circuitry. As it was the Castellan who gave the device to the Master, he is arrested and his rooms are searched -- and the forbidden Black Scrolls of Rassilon are located. The Scrolls self-destruct once the casket containing them is opened. The Castellan protests his innocence but Borusa orders the guards to mind-probe him to learn the truth. Obviously terrified, the Castellan is apparently shot while trying to escape. Borusa dismisses the Doctor and Flavia while he arranges the recovery of the Doctor's other selves from the Zone, but the Doctor isn't convinced that the truth has been revealed. The Castellan was stubborn and close-minded, but he was also fiercely devoted to his duty, and any mention of the Dark Times filled him with horror. Something else is going on...

As the Second Doctor and the Brigadier approach the Dark Tower, the Doctor explains that the Games were ended by Rassilon -- at least, according to the official histories. Rumours persist that in truth, his fellow Time Lords rebelled against his cruelty and walled him up alive in his Tomb... The Doctor locates an entrance to the cave systems beneath the Tower, which may lead to one of the legendary three entrances. But as they pass through the caves they are attacked by a yeti, apparently left over from the games. The Doctor tries to drive it off with a firework from his coat pocket, but this just maddens it and it nearly brings the wall down on the Doctor and the Brigadier while trying to get to them. They seem to be trapped behind a rockfall until the Doctor notices his torch flickering in a draught, and follows it to its source -- a door leading into the Tomb.

The Third Doctor and Sarah are pursued by a squadron of Cybermen, and as they near the cliffs they face a greater danger -- a Raston Warrior Robot, the most efficient killing machine ever devised. They are forced to stand utterly still as the motion-sensitive robot seeks them out, but at the last moment they are saved when the Cyberman patrol arrives in the pass. The Robot immediately sets upon them, and the lumbering Cybermen are no match for the sleek, incredibly fast killing machine. The Doctor and Sarah flee and manage to steal some of the Robot's spare armaments while it is occupied slaughtering the Cybermen. They then climb the cliffs to a ledge overlooking the Dark Tower, and the Doctor uses ropes from the Robot's armoury to lasso the roof of the Tower. He and Sarah slide over to a balcony on the Tower's roof, and enter through a hatchway.

The First Doctor and Tegan head directly for the Tower's main entrance, which opens via an entry-coder in the shape of a doorbell. A chessboard design is laid out in the main hallway, but the Doctor deduces that this is a trap; when he throws coins onto the design, nothing happens until they hit the halfway mark, at which point the entire board is strafed by deadly electric bolts. The Master arrives with the Cybermen in tow, and the Doctor and Tegan hide while the Master calmly crosses the board in complete safety. But when the Cybermen try to follow him they trigger the trap and are blasted down, and although the Cyber Leader survives, the Master takes one of the dead Cybermen's guns to dispose of him. He then taunts the Doctor, telling him the solution is as easy as pie, and departs into the depths of the Tower. The Doctor realizes that the Master was in fact referring to the Greek letter pi, and uses the formula to calculate the safe path across the board.

Back at the TARDIS, Turlough and Susan hear thumping noises outside, and activate the scanner to reveal that they've been located by a patrol of Cybermen. They are unable to move the TARDIS, which is held in place by a force field from the Dark Tower, and are forced to watch helplessly as the Cybermen plant bombs around the TARDIS, preparing to break in -- or destroy it in the attempt.

As the Doctors approach the Tomb at the heart of the Tower, the dormant mind of Rassilon sends out ghostly feelings of dread and terror. The Third Doctor scouts out the corridor ahead to reassure the alarmed Sarah, and is attacked by spectral versions of Liz Shaw and Mike Yates -- but he sees them for what they are and escapes, and the phantoms vanish into thin air. The Second Doctor and the Brigadier also encounter the shades of Jamie and Zoe, who beg them to go back -- but when Jamie identifies the Brigadier by name, the Doctor realizes that they're not real, as their memory of the Brigadier was erased when they were returned to their own time after his trial. They continue onwards to the Tomb of Rassilon, where the first three Doctors are reunited, and their companions share stories. Meanwhile, the Doctors try to decipher the ancient Gallifreyan script on a tombstone near the plinth where Rassilon lies in eternal sleep.

The Fifth Doctor returns to the Council chambers to speak of his doubts with Borusa, only to find the chambers empty -- and Borusa was not seen leaving. The Doctor searches the chambers and discovers that the harp in the corner, when played to a certain tune, opens a secret panel in the wall -- to reveal Borusa, the dark-clad Player, waiting for him over the controls of the Timescoop and the gameboard representing the Death Zone. Borusa has ruled Gallifrey behind the scenes for lifetimes, but this is not enough for him; clues in the Matrix left by Rassilon have led him to deduce that Rassilon discovered the secret of Immortality and set a task for his inevitable successor. Borusa sent the Doctors into the Zone to clear the way for him; now he intends to claim the prize of Immortality and become President Eternal. He uses the Coronet of Rassilon to amplify his willpower and take over the Doctor's mind.

The other Doctors translate the inscription and learn the secret of the Game -- but what does the motto mean, "to lose is to win and he who wins shall lose"? The Master arrives and tries to gun down the Doctors and claim Immortality for himself, but he's forgotten about the companions -- and the Brigadier surprises him and enables the Third Doctor to knock him out. The Doctors tie up the Master and the Third Doctor releases the force field which has been holding the TARDIS, enabling Turlough and Susan to escape seconds before the Cybermen detonate their bombs. But as they arrive in the Tomb area, Borusa and the Fifth Doctor transmat in, and Borusa uses the Coronet to freeze the companions in place; they will be unable to move until Borusa commands it.

The First, Second and Third Doctors combine their willpower and break the Fifth Doctor free of Borusa's control, but before they can deal with him personally the shade of Rassilon appears above his bier, offering his prize to the victor of the Game. Borusa claims his right to Immortality, and the Doctors protest -- except for the First, who urges Rassilon to give Borusa what he wants. Rassilon does so -- and the Immortal Borusa finds himself trapped on Rassilon's bier, frozen into stone for eternity. Rassilon releases the Master back into the timestream, promising he will face punishment in his due time, and frees the Fourth Doctor from the Time Eddy before returning to sleep. The First Doctor explains that he worked out what the motto meant; the Game was a trap to lure potential tyrants to the Tomb and dispose of them, thus ensuring that Gallifrey was never ruled by an Immortal dictator.