The Invisible Enemy is the second serial of the 15th season of the British science fiction television series Doctor Who, which was first broadcast in four weekly parts on BBC1 from 1 to 22 October 1977. The serial introduced the robot dog K9, voiced by John Leeson. In the serial, an intelligent virus intends to spread across the universe after finding a suitable spawning location on the moon Titan. Plot Some human space travellers are cruising near the outer planets of the solar system with their ship on autopilot. The TARDIS is travelling through the same region. The crews of both ships are infected by a sentient virus which chooses The Doctor to be the host of its "mind," the Nucleus of the Swarm. The Nucleus declares Leela a reject and orders her killed. The Doctor manages to break free of his infection and tells Leela how to get the TARDIS to the nearest medical centre. At the medical station, the Doctor's doctor, Professor Marius, introduces the group to K9, a robotic dog he made to replace the real dog he had to leave on Earth. Leela and the Doctor decide to create clones of themselves, which will then be shrunk and inserted into the Doctor. There they will destroy the Nucleus and escape through a tear duct. In the meantime, Leela and K9 fight off the infected staff of the hospital. The plan goes awry, allowing the Nucleus to escape and become human sized. The Nucleus and the infected staff leave for Titan Base so the Nucleus can spawn. The Doctor realises he is cured since Leela's clone introduced her immunity factor into his bloodstream. He replicates it and gives it to Prof. Marius. The Doctor, Leela, and K9 proceed to Titan Base in the TARDIS. They fight off the infected humans, but are again without sufficient weaponry to destroy the Nucleus, or its many children, which are about to hatch as "macro-sized" beings, like the newly macro-sized Nucleus. The Doctor jams the door they are behind and rigs a gun to fire into a cloud of oxygen gas he is releasing and escapes. As intended, when the Swarm finally forces open the door, the blaster fires, igniting the oxygen in Titan's methane atmosphere and destroying the Swarm and the base. When they return to the hospital, they thank Prof. Marius for the use of K9, who has ably