Joseph Cotten stars in A Blueprint for Murder as Whitney Cameron, a man who harbors a deep and abiding affection for his pretty sister-in-law Lynne Cameron (Jean Peters). When one of Lynne's stepchildren dies, the autopsy indicates that the child was poisoned. Despite his feelings for Lynne, Whitney can't help but feel that she was responsible for the child's death--and that she intends to go on killing until she becomes sole heir of her late husband's estate. The "moment of truth" occurs during a climactic ocean voyage which consumes nearly a third of the film. A Blueprint for Murder was written and directed by Andrew L. Stone, whose legendary insistence upon "realism" at all costs was such that it's a wonder he didn't persuade Peters to commit a real murder.
by Hal Erickson synopsis