A wealthy author's second wife begins to suspect that her 12-year old stepson may have murdered his mother, who mysteriously died in a bathtub accident.

The Beast in the Cellar is set in Lancashire in England during the early 70's where two elderly sisters name Ellie (Beryl Reid) & Joyce Ballantyne (Flora Robson) live together in their large house just outside the small rural town of Littlemead, one day Ellie rushes home to tell her sister that a soldier from a nearby Army camp has been brutally murdered. At first they both wonder who the killer could be until Ellie goes down into their cellar where they have kept their brother Steven (Dafydd Havard) bricked up for the past 30 odd years & discovers that he has dug a tunnel & escaped, obviously putting two & two together they feel Steven was responsible or the murder. The fact they find another dead soldier in their shed also has something to do with their thinking. Anyway, the beast is loose, on a murderous rampage & no-one is safe...