Doyle Gillette is in his last semester of law school and planning to marry his fiancée and graduate journalism student Kari McCrinnon. One day an anonymous message turns up on his answering machine one day saying, "You can't marry the girl you're engaged to. She is your sister."

Doyle's roommate, Lon, a fellow law student, says he'll help check things out. What they discover is that Doyle is not his father's biological son and the same is true for Kari.

Although they grew up only a few miles apart, Doyle and Kari went to different schools as children and never met until they were in college. Still, the more Doyle uncovers the more it seems likely that the mysterious caller may have known what he was talking about. While legally they are not related, can Doyle and Kari still marry if the facts reveal an unknown genetic link? It's a problem with both legal and moral implications.