With a relaxing few days in mind, Miss Emily Charters signs up for a four-day seniors' cruise of the Great Lakes organized by her church. In the company of her friend and housekeeper, Lynn Francis, Charters and the other seniors set sail from Chicago, but while the waters on the lake are calm and predictable, her fellow passengers are not. However, a little tension is the least of the problems when one of her fellow passengers is found horribly murdered in her cabin.

While the police investigate, the ship remains at port, leaving Emily stranded until her friend, Chicago Police Detective Jeremy Ransom, comes to her rescue. Together they set out to solve a most perplexing puzzle of a murder committed without seeming opportunity or plausible reason, not mention right under Emily's nose, in Fred Hunter's Ransom at Sea.