It follows Thorfinn, an Icelandic boy living in the early 11th century, who—after enduring a personal tragedy—sets out on a tale of revenge. Or at least, that’s how things initially appear.

Roughly 1,000 years ago, the story goes, a Viking trader and adventurer named Thorfinn Karlsefni set off from the west coast of Greenland with three ships and a band of Norse to explore a newly ...