An international criminal named Jacmel (Kinski) enlists Ruth Hopkins' maid Louise (George) and chauffeur Dave (Reed) in a scheme to kidnap her asthmatic ten-year-old son Philip (Holcomb) for ransom. As the plot begins to unfold, Philip has just brought home a snake from a local importer, unaware that his new pet has been accidentally switched with a deadly black mamba destined for a toxicology lab. The lab reports the mix-up, and a police officer is dispatched to the Hopkins residence, only to be shot by the panicking chauffeur. The London townhouse is surrounded by police, trapping the criminals, the child, and his grandfather (Hayden) inside with the mamba, which is now loose in the ventilation system.