Yuuto Suoh is a fourteen-year-old junior high student who decides to test an urban legend at the local Tsukimiya Shrine alongside his childhood friend Mitsuki. When he uses his smartphone to take a selfie with the shrine's divine mirror, a strange phenomenon transports him to a different world. This realm is called Yggdrasil, a land in the late Bronze Age around 1650 BC that is steeped in Norse mythology and divided into warring clans.

In the two years following his arrival, Yuuto uses the modern knowledge stored on his solar-powered smartphone to rise from a helpless outsider to the patriarch of the Wolf Clan, the family that took him in. He implements technologies and tactics far ahead of their time, including iron forging techniques,弩 (crossbows), stirrups, and military strategies he studied from texts like The Art of War and The Prince on his phone. Despite possessing no superhuman abilities himself, his strategic mind and commanding presence earn him the loyalty of several Einherjar, which are magic-wielding warrior maidens who swear fealty to him through a ritual called the Chalice Oath.

The main cast includes Felicia, Yuuto's sworn younger sister and副官 (adjutant) who wields a whip and was the one who inadvertently summoned him. She possesses the rune Faceless Retainer, which grants her a wide range of skills. Sigrun is his sworn daughter and a fierce warrior known as the Strongest Silver Wolf, whose rune enhances her physical strength to superhuman levels. Ingrid is a genius blacksmith with the rune Forge of Swords, who uses Yuuto's knowledge to revolutionize the clan's weapon production. Other key figures include Linnea, the former patriarch of the Horn Clan who becomes another sworn sister, and the twin sisters Albertina and Christina from the Claw Clan, who serve as an assassin and a spy, respectively. Throughout his conquests, Yuuto's ultimate goal remains to find a way back to Mitsuki in the modern world, though the warrior maidens competing for his affection complicate his mission.

A major early arc involves the Wolf Clan's desperate defense of its capital, Yalfeld, against a combined assault from the Claw, Fang, and Gray clans. Yuuto secures a decisive victory, after which the current patriarch steps down, leading to tragedy when a jealous clansman murders him. This event hardens Yuuto's resolve as he formally takes command. In a subsequent war against the Claw Clan, Yuuto earns the fearsome nickname Fenrir, the Notorious Wolf, after he burns a conquered town to the ground and relocates its entire population. Later, during a battle against an alliance of the Panther and Thunder clans, a mysterious magic known as Fimbulwinter banishes Yuuto back to modern Japan. He reconciles with his estranged father before deciding to return to Yggdrasil with Mitsuki to save the Wolf Clan from its desperate situation.