Doctor Who & The Invasion Of Time, #35 By Terrance Dicks (Target, 1984).

The Invasion of Time is the sixth and final serial of the 15th season of the British science fiction television series Doctor Who, which was first broadcast in six weekly parts on BBC1 from February 4 to March 11, 1978. It features the final appearance of Louise Jameson as the companion Leela.

In the serial, the Vardans break the defences of Gallifrey to allow the Sontarans to invade and control the power of the Time Lords.