'The Invisible Enemy' (4 Parts) Story Code '4T' by Bob Baker and Dave Martin

A space-borne intelligent virus known as the Swarm infects the crew of a space shuttle en route to Titan, one of the moons of Saturn. It also manages to infiltrate the TARDIS flight systems as the ship passes through it; the Doctor is infected, but is unaware of its presence in his body. The TARDIS then materialises on the Titan base, where Doctor and Leela discover that the human occupants of the refuelling station have been completely taken over by the virus. As the Doctor collapses as a result of the infection, he manages to give Leela the co-ordinates of the Bi-Al Foundation, a hospital on a nearby asteroid. Arriving at the hospital, Leela meets Professor Marius, and his robot dog-like computer K-9. In order to cure the Doctor, the Professor clones the two time travellers, miniaturises them using the TARDIS's relative dimensional stabiliser, and injects them into the Doctor's body, in the hope that they can find and destroy the virus Nucleus. But the plan goes wrong, and the Nucleus, instead of the clones, escapes from the Doctor's body, where it is unwittingly enlarged to human size. Marius is infected, and the Nucleus arranges to be taken back to Titan, where breeding tanks have been prepared for it. Now cured of the virus's influence, the Doctor, with the aid of K-9, sets a booby trap that blows up the breeding tanks, killing the nucleus, and freeing everyone from its influence. As a parting gift, Marius gives K-9 to the Doctor.

Tom Baker (Doctor Who), Louise Jameson (Leela), Voice of K-9 (John Leeson), Michael Sheard (Lowe), Brian Grellis (Safran), Edmund Pegge (Meeker), Jay Neill (Silvey), Anthony Rowlands (Crewman), John Leeson (Nucleus Voice), Frederick Jaeger (Professor Marius), Roy Herrick (Parsons), Elizabeth Norman (Marius' Nurse), Nell Curran (Reception Nurse), Jim McManus (Opthalmologist), Roderick Smith (Cruikshank), Kenneth Waller (Hedges), Pat Gorman (A Medic), John Scott Martin (Nucleus)

Produced by Graham Williams
Directed by Derrick Goodwin

TX:
Part 1 - 1st October, 1977 @ 6.20pm - 6.45pm
Part 2 - 8th October, 1977 @ 6.05pm - 6.30pm
Part 3 - 15th October, 1977 @ 6.10pm - 6.35pm
Part 4 - 22nd October, 1978 @ 6.10pm - 6.35pm

Notes:
*Featuring the Fourth Doctor, Leela and introducing K-9

*Working titles: 'The Enemy Within', 'The Invader Within' and 'The Invisible Invader'