A trio of criminals (Klaus Kinski, Oliver Reed, and Susan George) kidnap a boy and hold him and his grandfather hostage. But they soon find out that, due to a mix-up, the boy’s pet snake was accidentally switched with a deadly black mamba at the pet store. But the snake soon escapes, putting everyone’s lives in danger.

And in the end…

After the black mamba kills George and Reed, Kinski and the snake have a final battle, killing each other in the process. But unknown to the boy and his family, the snake was female and laid eggs in the family’s heating duct, and they are hatching…

Thanks Toplar Scarre!