An unknown entity uses the Time Scoop to bring several of the previous incarnations of the Doctor; his former companions Susan Foreman, Sarah Jane Smith, and Brigadier Lethbridge-Stewart; and his enemies the Daleks, the Cybermen, a Raston Warrior Robot and a Yeti, from their respective time streams into the Death Zone on Gallifrey. The entity's attempt to grab the Fourth Doctor and Romana ends up trapping the two in the time vortex. The Fifth Doctor senses the disruption of his own timeline, and with his own companions Tegan and Turlough, travels to Gallifrey via his TARDIS, also ending up in the Death Zone, unable to travel farther with the TARDIS due to a force field projected by the Tomb of Rassilon, the tower at the centre of the Death Zone. The various Doctors lead their companions towards the Tower while avoiding the hostile forces.
In the Citadel on Gallifrey, the High Council of Time Lords have also detected the disturbance in the Doctor's timeline and the power drain from the Time Scoop, and Lord President Borusa has the Master summoned, to help rescue the Doctor, offering him a new set of regenerations and a pardon if he succeeds. The Master accepts, and is given a recall device by the Castellan and a copy of the High Council's seal before he is transmatted to the Zone. The Master encounters the Third Doctor, who dismisses him and accuses him of making the seal himself, before finding the Fifth just as they are surrounded by Cybermen. The Master is knocked out by a Cyberman's gun firing, and the Doctor finds and uses the recall device to return to the Citadel. When the Master awakes, he makes a pact with the Cybermen to lead them to the Tower.
As the other Doctors and companions converge on the Tower, the Fifth Doctor discovers the recall device given to the Master included a tracking signal to lead the Cybermen to him. The Castellan is found to possess the forbidden Black Scrolls of Rassilon, and dies while attempting to escape an invasive mind probe. When the Doctor returns to the High Council's chamber to report, he finds Borusa missing, and soon discovers a secret room with Borusa at the controls of the Time Scoop. Borusa reveals he seeks to be the President Eternal of Gallifrey and needed the Doctors to disable the force field over the Tomb in order to gain immortality from Rassilon's Ring and rule forever. Borusa uses his headgear, the Coronet of Rassilon, to compel the Doctor to do his bidding. Meanwhile, the Master meets the and Tegan and rids himself of the Cybermen by letting them fall victim to the Death Zone's traps, before killing the Cyberleader with one of his subordinates' guns.