Another adventure for everyone's favourite time-traveller. Answering the Brigadier's space/time telegraph, the Doctor (Tom Baker), Sarah and Harry arrive in the village of Tulloch, near Loch Ness. A series of attacks have taken place on local oil rigs, and many are blaming the legendary monster. The Doctor discovers the creature to be the Skarasen - cyborg pet of invading aliens the Zygons. Their own planet having been destroyed and their spaceship crippled, these deadly shape- shifters are now intent upon taking control of the planet Earth.