The man-like beast that inspired the legend of the Norse wolf of destruction seeks out purpose in his eternal life by finding people to protect. But when that person inevitably dies, he violently takes out his grief and anger on other humans.

As his latest friend is on her deathbed he is urged to look for someone else to rely on as she is fading. He finds two children, twin sisters who are only seven years old. He befriends and protects them from the many dangers that the twins get themselves into. The parents of these kids think that the stories coming from their daughters are about a nonexistent friend they have made up from a traumatic event from their past. But the monstrous friend is real, and his name is Fenrir.