While returning to Los Angeles at night, private investigator Mike Hammer's (Ralph Meeker) car is flagged down by a woman named Christina (Cloris Leachman). She tries to evade Hammer's questions about where she is escaping from - barefoot and wearing only a trenchcoat - but he learns that she is from a nearby asylum. Nonetheless, he takes her through a roadblock. A few miles beyond, after a stop for gas at which she tells him that, should anything happen, he is to "remember me." Hammer's car is run off the road. Hammer is semiconscious while Christina is tortured and killed, and he is thrown clear when his car is pushed off a cliff. Hammer returns to consciousness in a hospital, where his secretary Velda (Maxine Cooper) and a detective of his acquaintance (Wesley Addy) inform him that a Federal investigators' board wants to question him. Their interest and Christina's cryptic message prompt him to ignore all warnings and begin his own investigation. He follows up a number of disconnected leads, all of which point to a conspiracy against a murdered scientist named Raymondo, organized by a lockal gangster, Carl Evello (Paul Stewart). The conspirators attempt to buy Hammer off with a conciliatory phone call and a new sports car, from which he has his mechanic (Nick Dennis) remove two bombs. When he visits Evello's house, Evello's sister Friday (Marian Carr) tries to seduce Hammer, and after subduing Evello's thugs, he is offered money by Evello. Hammer refuses; he traces Christina's roommate, Lily Carver (Gaby Rogers), and hides her in his apartment. Before he can go further, Nick the mechanic is killed and Hammer is abducted by Evello's men. At the gangster's beach house he encounters Dr. Soberin (Albert Dekker), whose voice Hammer recognizes as that of Christina's killer. Overcoming the influence of pentathol administered by Soberin, Hammer overpowers Evello and kills one of his men then returns home to find Velda missing, but with Lily Carver he decodes Christina's message through a poem by Rossetti. From a morgue attendant, Hammer obtains a key which leads to a locker containing what Velda had dubbed "the great whatsit." He leaves it in the locker but goes out to find Carver gone. At his apartment, the police are waiting; he is told that the box contains radioactive material being sought by foreign agents. When the locker is found empty, Hammer again follows his own lead to a patient of Soberin and gets Soberin's address from a prescription bottle. Arriving at Soberin's beach house, Hammer discovers that Lily Carver has killed him to gain sole possession of the box. Carver shoots Hammer and opens the container. The radioactive material sets her on fire and begins a chain reaction. Hammer struggles through the house, finds Velda, and frees her. Together they stumble into the surf as the house explodes.