The frost giant king Thrym once stole the Holy Sword Excalibur and hurled it into Urdr's Spring, severing the World Tree's roots and freezing Jotunheim into a dead realm. Kirito, Asuna, Leafa, Silica, and the rest of the ALfheim Online party descend into that frozen world to pull the sword back out, aided by a giant named Freyja who wants her family's stolen heirloom back. This manga adapts the Calibur side story, a self-contained fantasy-dungeon arc set between the Phantom Bullet and Mother's Rosario storylines.