International criminal Jacques Müller, Klaus Kinski and his girlfriend Louise Andrews, Susan George, hatch a plot to kidnap Philip Hopkins, Lance Holcomb — the teenage grandson of Howard Anderson, Sterling Hayden, a retired hunter who owns a vast hotel empire. Louise goes undercover as a maid working for Philip's mother Ruth Hopkins, Cornelia Sharpe, and she seduces her chauffeur Dave Averconnelly, Oliver Reed, convincing him to help pull off the kidnapping. On the day of the scheme, Müller baits Howard and Ruth into leaving the house, while Louise and Dave seize the boy.
Philip leaves briefly to fetch a pet snake, an errand that sets off a dangerous mix-up: a black mamba intended for toxicologist Dr. Marion Stowe, Sarah Miles, is swapped into the scene. When Howard returns early, the venomous snake is released and bites Louise in the face before it vanishes into the building’s ventilation. Müller and Dave then take Howard and Philip hostage, and Louise dies from the mamba’s venom, a shocking turn that heightens the tension.
Dr. Stowe quickly informs the police, and Commander William Bulloch, Nicol Williamson, arrives at the Hopkins residence. He learns about the hostages and insists on a strict stance, refusing Müller's demand for transportation and a ransom paid in various currencies. With Dr. Stowe’s arrival carrying anti-venom, Bulloch and the medical expert warn both sides about the black mamba. Müller tries to mislead Bulloch by claiming Louise is still alive, and he forces Dr. Stowe to come to the front door with the anti-venom so she can be used as leverage.
As the standoff unfolds, Bulloch uncovers a secret entrance to the cellar, and he and two officers prepare to move in. Dr. Stowe suggests cutting the central heating to drive the mamba into a temporary coma, a tactical maneuver that creates a narrow window for action. Dave and Howard ascend to the cellar at the same moment that Bulloch and his team push through, but the snake retaliates: Bulloch fires, injuring Dave, and the mamba attacks, forcing the investigators to retreat while Howard escapes back to the main floor. Dave is ultimately overwhelmed by the reptile and dies.
Müller then tries one last coercive ploy, presenting Bulloch with a severed finger and falsely claiming it belongs to Dr. Stowe, a ruse meant to break the stalemate. Bulloch, unable to reach the hostages through the cellar, agrees to hand over the confiscated getaway car, but he still demands to see the captives. Müller brings Dr. Stowe onto a balcony and compels her to disclose that Howard and Philip are safe, a move that buys him time. Howard and Philip notice the lurking mamba and deliberately provoke it to strike Müller, who stumbles into the line of the snake while grappling for control.
Howard pulls Dr. Stowe to safety as police snipers zero in on Müller and the reptile, delivering a volley of shots that end Müller’s plan and topple the snake off the balcony. With the hostages freed, Ruth runs to embrace Philip in a quiet moment of relief. The final shot reveals a troubling detail: the black mamba had laid an egg in the vents, and that hatchling quietly slithers away, leaving a lingering sense of danger behind.
