The Doctor and the Brigadier attempt to rescue Jo and Bert from the mine but Global Chemicals prove distinctly uncooperative.

The Doctor and Jo manage to escape from the mine with a maggot egg but Stevens is determined to stop them analysing it.

The Brigadier calls in a full UNIT force to deal with the giant maggots while the Doctor infiltrates Global Chemicals to find out what is really going on.

The Doctor manages to escape from BOSS with help from Yates and discovers Cliff has been infected by a giant maggot.

The Doctor attempts to destroy the giant maggots and cure Cliff but the real threat is BOSS, who is poised to take over all the world's computers.