The Doctor changes form again. Before he has fully re-covered, something of colossal strength steals a secret weapon, and the convalescent Doctor starts to investigate.

The Doctor and Sarah Jane become convinced Think Tank are behind the electronic component thefts but are unable to stop them carrying out another raid.

Sarah Jane and Kettlewell infiltrate an SRS meeting to try and find out what Miss Winters is up to but there is a surprise in store.

UNIT attempt to get into the bunker to stop Miss Winters setting off the world's nuclear missiles but first they have to get past the K1.