On the planet Zeta Minor, an expedition from a neighboring planet is doomed. Their ship is unable to lift off from the surface, and something is stalking and killing the crew one by one. The TARDIS arrives and the Doctor and Sarah offer their help, but they’re also suspected of causing the difficulties. The Doctor discovers that an attempt to bring a sample of antimatter back has attracted the unwelcome, but instinctively protective, attention of Zeta Minor’s native antimatter life forms. Worse yet, Professor Sorenson, hell-bent on keeping the sample aboard, continues his experiments with antimatter, slowly transforming himself into a hybrid matter-antimatter creature with no control over his actions.
written by Louis Marks
directed by David Maloney
music by Dudley SimpsonGuest Cast: Terence Brook (Braun), Tony McEwan (Baldwin), Frederick Jaeger (Sorenson), Ewen Solon (Vishinsky), Prentis Hancock (Salamar), Michael Wisher (Morelli / voice of Ranjit), Graham Weston (De Haan), Louis Mahoney (Ponti), Haydn Wood (O’Hara), Melvyn Bedford (Reig), Mike Lee Lane (Monster)
Broadcast from September 27 through October 18, 1975
LogBook entry & review by Earl Green
Review: Okay, you’re just going to have to ignore the “science” of this four-parter. It really isn’t very airtight, and yet this story contains some of the best cliffhangers of the early Tom Baker era. You’ll just have to take it for granted, to give one example, that exposure to antimatter doesn’t rip a man made of normal matter to parts in a moment of mutual annihilation, but instead just makes him a 
mad, hairy beast – as in just-this-side-of-werewolf hairy. The special effects are right out of the Pertwee era, and yet it all works – it’s got a nice “Ten Little Indians” atmosphere of growing dread to it. The exotic jungle “exteriors” are actually all studio-bound – a rare example of the BBC set and scenery designers and builders really getting to show off on something other than a reproduction of period architecture.
You might notice Space: 1999’s Prentis Hancock taking a break from his Moonbase Alpha duties – he was a friend of this story’s director, and I’ve always wondered if Hancock’s curious and conspicuous absence from one particular Space: 1999 episode had anything to do with his appearance here. To a certain degree, he’s really playing the same authoritatian character. This guy gets around.