When an old man comes to stay with Oliver's parents, Oliver is astonished by the man's acute phobia of fleas and rats. Determined to discover the source of the old man's terror, Oliver is drawn to a demolition site, where workmen are suffering appalling mental injuries. Oliver realises that something of enormous, terrifying power has been unearthed by the workmen but before he can find out more, a terrified scream comes from the cellar of his house. Thinking the screams to be from the old man, Oliver plunges into the cellar...to find himself in plague-ridden London, trapped in the body of a boy, surrounded by plague victims.