Tom Baker
The Invisible Enemy

A spacecraft en route to the Titan base is infected by a virus, and the same virus - the Swarm - also attacks the TARDIS. The Doctor is infected by the Nucleus of the Swarm. Landing at the Titan Base the Doctor and Leela are threatened by the crew. The Doctor and Leela manage to get away to a hospital for the Doctor - the Bi-Al Foundation.

Professor Marius is able to clone the Doctor and Leela, minaturise them, and inject them into the real Doctor. The hope is that they will be able to find the infection and destroy it.

The Swarm nucleus manages to get out and is enlarged to human size. It arranges for transport back to the Titan base where breeding tanks have been prepared for it.

The now cured Doctor, along with the help of Professor Marius's mobile computer K-9, manages to set a trap which will blow up the breeding tanks. As a parting gift Professor Marius leaves K-9 in the care of the Doctor.