The medical center mounts a defense against alien forces trying to take the comatose Doctor while clones of Leela and the Doctor go microscopic so they can hunt down the intelligent infection that's in his head.
Did You Know?
found the confrontation with Nucleus difficult to film, due to the creature's design unfortunately resembling a prawn. He would break character by laughing, even singing 'Prawn Free'. At one point, he blurted out, "How am I supposed to act to a fucking prawn?" See more
[as a bright light noisily shoots past them in the Doctor's head
What was that?
Ah, just a passing thought.