John Leeson stars in this exciting novelisation of a classic adventure for the Fourth Doctor - and the introduction of K9. A mysterious cloud drifts menacingly through space, and the Doctor becomes infected with the Nucleus of a malignant Virus that threatens to destroy his mind. Meanwhile, on Titan, human slaves prepare the Hive from which the Virus will swarm out and infect the universe. In search of a cure, Leela takes the Doctor to the Bi-Al Foundation, where they make an incredible journey into the Doctor's brain in an attempt to destroy the Nucleus. Can the Doctor free himself...