A British woman visits her husband at a Mexican mine that he is attempting to reopen and discovers that the workers refuse to enter the mine because of an ancient curse. The couple enters the mine to prove there is no danger and inadvertently releases a demon that possess people's left hands and forces them to behave in a suitably diabolical manner. The only way for a possessed person to free themselves from this torment is to cut off his/her left hand, after which it scurries away in search of its next victim.