Frank settles in as commanding officer, only to have a new one appointed over his head, one that, to his chagrin, fits in very well.

Scotland, the near future. Something is smashing oil rigs off the Scottish coast, and UNIT have been called in to investigate. The Doctor discovers a colony of shapeshifting Zygons and their cyborg weapon, the Skarasen, and in the process solves one of Scotland's greatest mysteries...