Kate Graham is unhappily married to a very wealthy man. As the marriage disintegrates, her husband is found murdered, and Kate stands to inherit his considerable fortune.

A few days later, Michael Matheson, a mysterious stranger, takes her on a wild cab ride and tells her she owes him ten per cent of the inheritance. "I killed your husband for you," he explains. "I'm going to insist on being paid for my services."

And there's the catch: if Kate refuses to pay, Matheson will frame her for the murder. Matheson is confident she'll pay, however: "Everyone always pays."

Kate, a highly moral woman, decides to fight back, and Matheson frames her for the murder, as threatened. She finds herself running from the police and desperately trying to find Matheson, a man no one believes exists.

As the story progresses, Kate discovers that Matheson has killed many times before - he's a killer for hire, but he kills in advance, on spec - and in order to find him she has to track down the next victim on his list.

She finds the intended victim, but he doesn't believe her story, and his lack of faith costs him his life. Kate is now implicated in a second murder.

With the authorities closing in on her, Kate unmasks Matheson's true identity, fights him on his own turf, and finally exonerates herself.

Matheson ends up in prison, but he's not finished with her yet...