Twenty years earlier,aged seven,young Henry Knight saw his father torn to pieces by a monstrous creature at Dewer's Hollow near their Dartmoor home. Now Henry has seen the footprints of a huge beast and suspects that the nearby Baskerville government research station is breeding mutant animals.

Sherlock and John travel to the moor where local lad Fletcher organizes tourist walks cashing in on the legend of Dartmoor's spectral hound. Using false I.D. the pair infiltrate Baskerville and are challenged by secretive Major Barrymore but rescued by sympathetic Dr. Frankland, a friend to Henry.

After Sherlock himself sees the monstrous creature he enlists the help of geneticist Dr. Stapleton to help him solve the mystery.