In a post-Ragnarök Minnesota, 13-year-old Valkyrie, Gunnr, must train the prophesied hero meant to save the dying realms: a scrawny boy in hiking boots who's only ever wielded a flashlight.

Desperate to join her parents' war against the remaining Ragnarök monsters, Gunnr—keeper of Odin's ravens and top Valkyrie in her division—is tasked with not only finding the prophesied hero but training him to fight. But Beowulf, the supposed warrior she finds hiding in the World Tree's tunnels, thinks horses are monsters and uses bear mace to win his first duel instead of engaging in combat.

With monsters tearing her home apart, Gunnr doesn't have the time to train a hero who refuses to kill. So, when she discovers Beowulf's strange gift of invincibility, they make a pact: he'll learn to fight and play the hero the world expects, and she'll be the one who actually destroys their opponents.

But as a deadly poison spreads through the World Tree from within, defeating monsters with swords and axes may not be enough. To save the realms from complete collapse, Gunnr will have to trust Beowulf and his flashlight to destroy the darkness, even if it means granting mercy to the creatures threatening the survival of all existence.