Doctor Who: The Defectors (198)

Jo Grant is shocked to find most of her colleagues are missing. Then she discovers that the Doctor has inexplicably changed.

But there's no time to worry about it, as she and her misplaced Time Lord friend are whisked to the mysterious Delphin Isle on a matter of national security. There, they encounter a disturbingly odd form of local hospitality and learn of a highly classified incident that took place during the Cold War.

Why exactly have they been brought here? And what is the truth concerning the bodies in the harbour and the vast project being undertaken beneath a cloak of secrecy?