- Terrorists in the process of kidnapping a child get trapped in a house with an extremely deadly snake.
- In London, the boy Philip Hopkins lives with his wealthy family and collects pets, encouraged by his grandfather Howard Anderson who is a famous hunter. When his mother Ruth Hopkins travels to meet her husband in Rome, Philip stays with Howard, the maid Louise Andrews and the chauffeur Dave. However, Louise and Dave have plotted to kidnap Philip with her lover Jacmel. The plan begins to fail when Philip goes to the pet shop to bring the snake he ordered, and the owner commits a mistake and delivers a dangerous black mamba instead. When the owner of the mamba, Dr. Marion Stowe, receives the pet snake, she learns the mistake and calls the police. One police officer goes to Philip's home and is shot by Dave. Now the police, under the command of the tough Commander William Bulloch, puts the house under siege. Meanwhile, the black mamba is free inside the house and bites Louise who dies.—Claudio Carvalho, Rio de Janeiro, Brazil
- International terrorists attempt to kidnap a wealthy couples child. Their plan comes unstuck when, a deadly Black Mamba sent by mistake instead of a harmless snake, escapes, and the terrorists and several hostages are trapped in the boy's London home. A tense evening is had by all as the snake creeps around the house picking off the various characters one by one.—R Lindsay
- A plot to abduct a young boy for ransom is foiled when the two kidnappers and their hostages are up trapped by the police in a posh London row house... along with a deadly black mamba snake running loose within and killing them off one by one.—Humberto Amador
- In London, England, international criminal Jacques Müller (Klaus Kinski) and his girlfriend Louise Andrews (Susan George) plan to kidnap Philip Hopkins (Lance Holcomb), the 10-year-old grandson of Howard Anderson (Sterling Hayden), a retired American hunter and the wealthy owner of an international hotel chain. Louise goes undercover as a maid working for Philip's widowed mother Ruth (Cornelia Sharpe) and seduces her chauffeur Dave Averconnelly (Oliver Reed), convincing him to help in the kidnapping.
On the day of the kidnapping, Müller tricks Howard and Ruth into leaving home while Louise and Dave kidnap the boy. But the kidnapping plans go awry when Philip leaves the row house briefly to retrieve a pet snake, which is accidentally swapped with a venomous African black mamba meant for toxicologist Dr. Marion Stowe (Sarah Miles). Howard returns home early, and the black mamba is released from it's container and bites Louise in her face three times before fleeing into the ventilation system.
Müller and Dave take Howard and Philip hostage, while Louise quickly dies from the black mamba's venom. Dr. Stowe contacts the police, having discovered the mix-up, and a police officer is dispatched to the Hopkins residence. The officer is shot and killed by Dave with one of Howard's hunting rifles, but the officer manages to call for backup before he dies. More police officers arrive, led by Commander William Bulloch (Nicol Williamson), and after learning about the hostages, Bulloch has the street sealed off and tries to negotiate with Müller, but refuses to give in to Müller's demands for transportation and one million in different currencies.
That evening, with the siege of the house underway, Dr. Stove arrives with a case of anti-venom and informs Bulloch of the black mamba inside the house. Bulloch and Dr. Stove warn both the kidnappers and hostages of the snake, and Müller lies that Louise is still alive after being bitten, and orders Dr. Stove to come to the front door with the anti-venom to treat her. Dr. Stove complies and is taken hostage.
Meanwhile, Bulloch discovers a secret entrance through the cellar, and he and two other officers try to enter. Inside the house, Dr. Stove suggests turning off the central heating source, as this would send the black mamba into a coma, but Dave and Muller refuse. Dave and Howard climb down into the cellar to look for the snake at the same time Bulloch and the officers enter. Bulloch shoots and injures Dave, and the black mamba suddenly attacks, forcing Bulloch and the officers to flee while Howard flees from the cellar and back onto the first floor. Dave is unable to escape due to his gunshot wound and is bitten and killed by the black mamba.
Müller gives Bulloch a severed finger, which he falsely claims belongs to Dr. Stove. Unable to enter the cellar with the black mamba inside the house, Bulloch gives in to Müller's demands and brings him his getaway car, which was previously confiscated. Bulloch demands to see the hostages, and Müller forces Dr. Stove onto a balcony, forcing her to tell Bulloch Howard and Philip are fine. Philip and Howard notice the black mamba emerging from the vent and allow it to attack Müller, who stumbles onto the balcony while grappling with the snake. Howard pulls Dr. Stove out of the line of fire, and police snipers proceed to shoot Müller and the snake multiple times, and Müller and the black mamba tumble off the balcony both dead. The hostages are rescued and Ruth embraces Philip. The final shot of the film reveals that the black mamba had laid an egg in the vents, which hatch and the newborn snakes slither off.
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