The heart beating in Clara Kane-Rivera’s chest belonged to a young man named Lucas Bennett.

Twenty-nine years old.

A high school history teacher from a small town in northern Wisconsin.

A volunteer firefighter.

A son, brother, fiancé.

A stranger whose final act of kindness had given Clara a second chance at life.

The Kane family had always known the donor was anonymous at first—protocol for organ donation. But months after the transplant, when ...