There are two story lines running through Once Upon An Island, both taking place on the same landscape, Manitoulin Island.
An ex-convict determined to get his revenge on society by committing unsolvable crimes. He has had plenty of time in prison to develop an ingenious and complicated strategy that includes collecting objects which determine whom he kills, where he kills and when. He has determined his next victim will be on Manitoulin Island.
At one of the local auction sales he befriends two elderly ladies and is invited to stay with one of them. He is absolutely wonderful to her, and she finds in him the son she never had. He gains the confidence of the people in the community, taking part in projects and social activities. After catching him in a bold-faced lie, the two elderly ladies decide to take the matter into their own hands and see what he is up to.
The true part of the story develops around characters and incidents from the past that are woven into the fabric of the mystery.