Transmitted originally: 27 September 1975 to 18 October 1975.
Sorenson becomes infected by the anitmatter and gradually transforms into a creature capable of draining the life from others. The Morestran commander, Salamar, becomes increasingly unhinged, attacks Sorenson with a radiation source, which causes him to multiply. The ship is soon overrun by the deadly creatures. The Doctor finds the original Sorenson, and takes him back to the planet in the TARDIS, throwing both him and the antimatter samples into the pool, fulfilling a bargain made with the inhabitants of the other universe to stop the interference. Sorenson reappears unharmed and is returned to the Morestran ship, which is able to leave the planet.
'Zeta Major' by Simon Messingham, a 1998 novel featuring the Fifth Doctor, Tegan and Nyssa is a sequel to this story.
OTHER FORMATS:
The Fourth Doctor Novelisations
Back to the Fourth Doctor Programme Guide
Back to 'Terror of the Zygons'.
Forward to'Pyramids of Mars'.