When a mysterious disease strikes the Cornish village of Pentruan, Doctor Howard Spence calls for help on Professor Julius Slade, a medical research scientist and his assistant, Ruth Pollard. But Ruth has a painful secret that complicates her relationship with Howard. Howard, Ruth and Professor Slade become involved in a desperate search for the cause of the bizarre laughter that is choking the life out of a middle-aged man and a teenage girl.