Turlough leaves in Planet of Fire when he is offered a chance to return home.
Peri joins in Planet of Fire.
The Doctor regenerates into his sixth persona, played by Colin Baker, at the end of The Caves of Androzani. Episode Details Warriors of the Deep The Doctor and his companions arrive on Sea Base 4, a 21st century underwater nuclear missile base. The Base comes under attack from Sea Devils and Silurians, and their prime weapon a fearsome creature called the Myrka. The Silurians plan to fire the base's nuclear missiles, provoking a full scale nuclear war which will destroy mankind. They find a small group of humans apparently, the only survivors of the human race barely surviving after the crash of their spacecraft.