Guy Haines, a tennis star who hates his wife, is approached on a train by a stranger, Bruno Anthony, who hates his father. Anthony offers a plan: each could kill the other's victim. No motive, no clue would link the two murders save the casual meeting of strangers on a train. Haines doesn't take the plan seriously, however, until his wife is suddenly murdered, and Anthony appears to demand that Haines keep his part of the bargain.