Set after The Sea Devils. Called in by a Scotland Yard pathologist to diagnose a deteriorating brain, the Doctor realizes that a new illness has been introduced into the population. But what are its causes? When three men in different parts of London fall into irreversible dementia and then die, he must find the link between them. He and Jo enter the seamy, dark side of sports competition when they investigate a possible link of the illness to experimental anabolic steroids. Eight Episodes.