Near the end of the 19th century, Prof. John Mayer and his assistant, the scientist Dr. Isabel Reed, accidentally invent a powerful ray. An alien travels to Earth to destroy the machine since it would be a threat to the universe. He initially possesses the mind of Thomas, a sex maniac and serial killer, to get close to Mayer during the demonstration of the machine to scientists and the military. Mayer's niece, Laura, meets an acquaintance of her uncle, the chemist Dr. Paul Rosten, and Mayer invites him to join the research. When Thomas gets close to Mayer, the alien also possesses his mind in order to destroy the device and his notes. Meanwhile, Thomas is not able to control his murderous impulses and attacks several women with a straight razor.

The main character is a nameless boy (Juan Jose Ballesta) who was taught to steal wallets by his absent mother. He is able to do the trick effortlessly, using his "earnings" to survive while he looks for his mom. As he runs into an antique store owner, he might have found a trace that will lead to her. First, however, he will have to pull off a few "jobs" for the store owner, and he can't do it alone: enter a nameless girl (Maria Valverde) who gets to learn all the secret techniques the young boy has been using all his life.