INVISIBLE ENEMY is the story that introduced robot dog K-9, who would re-appear in the new, revamped series episode 'School Reunion'. Also features 'K9 & Company' - the very first Doctor Who spin-off starring K-9 and Sarah Jane Smith, who would later feature in SARAH JANE ADVENTURES.
The Invisible Enemy
Starring Tom Baker and Louise Jameson
The TARDIS is infiltrated by the Swarm - a space-borne intelligence that wishes to spread itself across the universe - and the Doctor is infected by its nucleus. The ship then materialises on Titan, one of the moons of Saturn, where the human occupants of a refuelling station have also been taken over.
The Doctor eventually collapses as a result of his infection, but first manages to relay to Leela the coordinates of a local hospital asteroid. At the Bi-Al Foundation, based on the asteroid, Professor Marius clones the two time travellers, miniaturises the clones using the relative dimensional stabiliser from the TARDIS and then injects them into the Doctor's body in the hope that they can find and destroy the nucleus.
The plan backfires as the nucleus escapes from the Doctor in place of the clones and is enlarged to human size. The creature arranges for itself to be taken back to Titan, where breeding tanks have been prepared for it.
The Doctor, now cured of its influence, enlists the help of K9, Professor Marius's dog-shaped robot computer, and sets a booby-trap that results in the breeding tanks being blown up, killing the nucleus. Marius gives K9 to the Doctor as a parting gift.
K9 & Co.
Starring Elisabeth Sladen
The first Doctor spin-off, K9 and Company showcases the Doctor's two companions, Sarah Jane (Elisabeth Sladen) and robot K9, as they attempt to solve a bewitching mystery.