John Mannering (aka 'The Baron') is a retired jewel thief who is regularly consulted about cases by Scotland Yard. Now, however, he finds himself the chief suspect in a murder and robbery and is locked up in jail where, from his prison cell, he must find the answer to thirteen difficult questions in order to solve the crime and prove his innocence. Meanwhile, the real perpetrators are free and able to further complicate matters and make Mannering's task even more difficult.