Episode list

In Edwardian London, that scandalous investigator of occult phenomena, Mr Herbert Scott, and his associate, Father Felix Cromwell, are confronted by a case of possession. A young woman, suffering from unusually vivid dreams, seems also to exhibit signs of a completely separate personality. Another being is speaking through her, a being which claims not to be from Earth at all, a being which calls itself... Zygon.

Freddie Trewella has found a mermaid, beautiful and strange, washed ashore in a Cornish bay. He names her Vorvoren, and she comforts him in his harsh life of servitude to a zealot father. As they grow together, Vorvoren shows Freddie her secret power, to change her appearance into anything she desires. But Freddie has secrets of his own, terrible secrets, and soon Vorvoren's great adventure among the humans becomes a nightmarish fight for survival.

In an England gripped by Cold War paranoia, wounded MI5 operative Caldwell is itching to rejoin the fray abroad. So why are the service sending him to a remote Cornish island? Can the future of the world really be at stake? Will he be able to complete his mission without falling foul of the hideous monsters stalking the caves? And who exactly is the island's other new arrival: an operative of quite a different kind, known only as the Doctor?