The Doctor and Adric are summoned by the old Keeper of Traken when an ancient evil is unleashed on the peaceful world. Cassia is given over to a stone statue of one of the evil beings that landed on Traken--Melkur. Cassia's step daughter, the daughter of Counsel Tremas, Nyssa, helps the Doctor and Adric when they are blamed for a series of murders in the gardens. The force behind all this is the Master and the Melkur is his TARDIS, which he uses to seize the Keepership, killing the Keeper and Cassia. Adric and Nyssa carry out a plan of the doctor's and Tremas and stop the Master--who is in his old, wizened form, in between regenerations. The Doctor and Adric leave and don't know that the Master melds into Tremas. 1981 or 4950 AD.