The Doctor returns to Gallifrey after a suspicious meeting in space with a group of unseen aliens. When he arrives at the Time Lord Capitol he claims the position of Lord President which is his by right as he was the only candidate to the post following the assassination of the former President (see The Deadly Assassin). The Doctor commandeers an office and gives orders for it to be decorated with panels beaten from lead. Leela cannot understand what has got into the Doctor as he is behaving irrationally and completely out of character.

The unseen aliens watch as the Doctor sets events in motion to have himself inaugurated as President. He is 'crowned' with a device giving him access to the Matrix and the aliens attempt to pry into this repository of Time Lord knowledge, an intrusion which the Doctor prevents by a massive force of will.

The Doctor then arranges for the transduction barriers around Gallifrey to be destroyed by K-9, and when this is done his alien 'friends' materialise in the Panopticon. They are Vardans, beings which intend to take over Gallifrey.

The Doctor maintains his mind shield against the invaders as K-9 links with the Matrix to work out where their home planet is. He plans to place a time loop around the planet but must make sure the Vardans do not suspect him until he is ready. He banishes Leela to the wastelands of outer Gallifrey as she is in danger of unintentionally jeopardising his plans. There she meets up with a group of outlaw Time Lords who organise an attack on the Capitol to fight off the invaders. 

The Doctor finally springs his trap and the Vardans are banished. However, at almost the same time, Gallifrey is invaded by troopers from the Sontaran Special Space Service, who were, unknown to the Doctor, using the Vardans as a bridgehead to enable them to conquer the Time Lords.

The Doctor and Leela, with the help of a few of the Time Lords and a group of the outsiders, despatch the Sontarans one by one once the invaders have gained access to the TARDIS.

Realising that the Sontarans are after the Matrix, the Doctor uses knowledge extracted from it by K-9 to construct a forbidden disintegrator gun, activated by the Great Key of Rassilon. He uses this to kill the remaining Sontaran in the TARDIS and then confronts the Sontaran leader, Stor, in the Panopticon. Stor intends to destroy the Time Lords' power centre with a bomb, but the Doctor activates the gun as the charge detonates.

The Doctor survives the release of energy although his memory of recent events, including the fact that he has become President, is wiped. He decides to leave Gallifrey as he still has much to achieve in the Universe. Leela announces that she wishes to stay on Gallifrey with Commander Andred, one of the Chancellory guards, with whom she has fallen in love. K-9 also decides to stay with Leela.

As the TARDIS dematerialises, the Doctor pulls out a large box marked 'K9 MII'.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}