The young American Perpugilliam Brown brings to the TARDIS a mysterious object that her archaeologist step-father has found in a sunken wreck. Kamelion, the Doctor's robot friend of a thousand disguises, reacts to the object totally unexpectedly, with bewildering consequences for the TARDIS crew.

For Kamelion sends the Doctor and his friends to Sarn, a terrifyingly beautiful planet of fire. This strange world provides the key to Turlough's secret past – and once again the Doctor is pitted against the wily Master.