The Doctor arrives on Gallifrey, where he is accused of the assassination of the Time Lord President. Investigating with the aid of Co-ordinator Engin and Castellan Spandrell, he discovers that this is part of a plot hatched by his old adversary the Master.

Having used up all twelve of his regenerations, the Master is now a wizened husk. He is seeking to control the presidency in order to obtain the official regalia, the Sash and Rod of Rassilon, which are really keys to the Eye of Harmony, the source of all the Time Lords' power.