Safecracker Chuck Spillers is finally released from prison. He goes home to meet his wife Annie. He vows never to return to prison as he is tired of being locked up.
He shows Annie a brown paper envelope given to him by a prison visitor, an old man who said he knew what it was like to be locked up as he was once the inmate of a nazi concentration camp. The envelope contains a large wad of money. Having the address of his mysterious prison visitor, he goes to see him to say thank you.
The address happens to be that of a pet shop. The old man, Martin Blueck, is pleased to see him and tells Chuck that he knew he would come. As Chuck is about to leave, Blueck stops him and tells him to put the closed sign up as he wants to show him something.
Chuck is taken downstairs to the basement, and he is staggered to see many wild animals in their cages with the doors open. Blueck explains to him that the wild animals are not tamed, but just trained. The animals wait inside their cages until a buzzer sounds which alerts them to take their food.
Blueck tells Chuck that he needs someone that he can trust to look after his animals as he is going away for a few days. Chuck eventually agrees.
While Chuck cares for Blueck's animals, he notices a safe on the wall. He tells Annie about the safe and how it will allow them to pay some bills, if he should go back to his old safecracking ways, but she will have none of it.
Chuck simply cannot resist having a go at Blueck's safe. He manages to successfully open it, but as soon as he does, a trap-door opens and he falls into a cell. Not long after his fall, it becomes apparent that Blueck is aiming to use Chuck as an experimental subject in his advanced conditioning techniques.
Worried when Chuck doesn't arrive home that night, Annie pays Blueck a visit. Blueck tells her that he hasn't seen Chuck since he came out of prison. Annie spots Chuck's jacket and knows that Blueck is lying.
She tells a police inspector the tale, but he doesn't want to get involved until he has been missing for a lot longer.
Annie goes back to the pet shop and manages to make it inside. She finds Chuck. Annie wants to contact the police, but Chuck is against the idea as he will have to explain how he came to be there. Annie has to go back outside to retrieve a length of rope. As she lowers it to Chuck, Blueck pushes her into the cell. Now both Chuck and Annie are trapped.
The couple are now subjects of Blueck's experiment. They are almost driven to the brink of madness by the constant buzzing, but quickly discover a flaw in the operation of the force field and manage to escape. Chuck enters the rear of the pet shop and sees Blueck about to be attacked by a panther that has escaped from its cage. Blueck, trying to avoid being attacked by the panther, falls into the cell.
Chuck and Annie return home. When they enter the kitchen, they hear an ominous buzzing noise. Blueck meant for them to escape and now they are trapped in their isolated house with no-one to hear their silent screams.