A line of prisoners are going to Devil's Island and as they reach a dock, someone pulls one of them into the water. The guards pull the man out and reveal he's… James West. Beneath the dock, Artie assures the real prisoner that he's safe and Jim needs to get to Devil's Island.
The next morning, Jim and the other prisoners arrive as the Commandant' wife, Camille, looks out and notices Jim. Jim has orders to be taken to the Commandant and claims he's an undercover prison inspector to avoid immediate punishment. Camille makes herself up to look particularly attractive while Jim is taken to Commandant Gustave Mauvais, who introduces his henchman Couteau. Gustave identifies Jim as a writer who was prepared to write an editorial against Devil's Island and call for a trail against Gustave. Jim tries to approach Gustave who flies into a panic, as he's obsessed with cleanliness. Camille arrives to ask for fashion tips from her husband but he quickly realizes what she's up to. He dismisses her and then has Couteau demonstrate he has an iron leg that he can use to literally crush any resistance.
Jim is taken back out into the prison and put to work, and he secretly tries to locate a prisoner named Reed. The water wagon arrives and Jim knocks a prisoner down when he stops an older man, Lime, from getting his ration. Jim talks to the man who remembers that Reed was hauled off to the "Pit" over two weeks ago and is probably dead. Couteau notices him and knocks the man unconscious into the path of a roller. Gustave is there and unimpressed with Jim's suggestion that they get the man out of the way. Jim breaks free of Couteau and grabs Gustave, telling him to order the roller be stopped. Couteau stops it just in time and Gustave shoves Jim down to the guards and orders a flogging then confinement to the pit.
Jim is tossed into the pit and exchanges passwords with Reed to prove his credentials, then assures him he was sent to rescue the undercover agent who maintained his cover throughout his trial. Another prisoner in the pit, Le Fou, rants and raves and Jim assures Reed they will be escaping soon.
Up above, Artie has arrived as a guard "sentenced" to Devil's Island to work after being thrown out of the French Foreign Legion. Gustave is impressed by his fake record of treachery and prepares to take him on but notes there are two other applicants for the job. Artie is locked in a room with the other two and the last one standing gets the job. Artie makes short work of them with the aid of a gas grenade. Artie then fakes a much louder fight and emerges unscathed, much to Gustave's bemusement.
That night Artie fakes being drunk and taunts Jim and Reed in the pit, and drops down an egg before staggering off. Jim cracks open the egg to reveal the key to the overhead door. Jim climbs up and gets himself out, but the guards are waiting for him. They take him to the Commandant's house and a secret passage which leads to Camille's bedchamber. She wants him to help her escape and can provide a boat and money. She's interrupted when Gustave comes in and reveals her attempted escapes are a game he plays.
Gustave has Jim staked out along the path of fire ants and left to die. Meanwhile, Artie goes to shore and visits a boutique where the owner Miss Grimes is another operative. She has a boat ready for him and Jim. Artie then gets back to the island and cuts Jim free and they head for the prison. Artie brings Jim as a captured prisoner and takes him to the pit where they try to get Reed out while claiming he's to be taken to Gustave. Gustave shows up to ruin the plan thanks to Le Fou, who is an undercover guard.
Gustave takes Jim to the center of the compound and orders Couteau to crush him with his iron leg. Jim manages to bring the guard down and Artie provides cover fire with the grenade launch in his cane. Jim, Artie, and Reed make a break for it and Camille gets them to a secret passageway leading back to her boudoir. They escape out the window and she insists on going with them. They get to the beach, but Gustave and Couteau know about their plan since Couteau tortured the information out of her, and they're already waiting for them. The escapees row away but as Gustave prepares to shoot them, he stumbles into quicksand. Couteau ignores him and walks into the water and Gustave shoots him for his betrayal. Gustave sinks into the earth and a shark finishes off the guard captain.
The agents rendezvous with a boat and later back in the States at the Wanderer, Camille arrives to introduce them to her fiancée, a much older man who looks like her last husband.