Although Dartmoor jail is surrounded by treacherous swamps, 12 prisoners have managed to escape in the last 3 years. However none of them have ever been seen again. So P.I. Nash fears that all of them have been killed by the secret organization they were members of and starts to investigate. He clashes immediately with Inspector Cornwell from Scotland Yard, who's arrived at Dartmoor Inn near the prison, too, for an official investigation.