The TARDIS suffers a power loss on landing on an unknown planet. Eventually the Doctor and Sarah manage to get out of the TARDIS and explore the bleak surface. They eventually meet up with another an Earth Marine Space Corps party who had landed on Exxilon to obtain parrinium, a cure for a space plague that was spreading through the galaxy.
Another ship falls victim to the power drain, and the Doctor, Sarah and the humans go to assist. The occupants are Daleks who instantly open fire - only to find their energy weapons also do not work. The Daleks and the humans form an uneasy alliance with each other.
Sarah finds a huge white city, and is captured by the savage Exxilons who plan to sacrifice her for defiling their city. The Doctor manages to save her and they escape to the underground, where they meet a friendly Exxilon, Bellal, who tells them about the city that was built by their forefather, and became their downfall.
The Doctor, and the Daleks, realise that the city's beacon is causing the power drain. The Daleks plan to destroy the beacon with a bomb, whilst the Doctor, Sarah and Bellal, followed by Daleks, destroy the city from within. With power restored the Dalek alliance is at an end. They planned to steal the parrinium so that the plague would spread. Instead one of the humans, Galloway, takes a bomb on board the Dalek ship and detonates.
