In the labyrinth city of Grey Neighbourhood, also known as the Dungeon City, crime, debauchery, and death are commonplace. The city is built around a massive dungeon filled with monsters and treasure, attracting adventurers seeking fortune. The sole shining light in this wicked place is Arwin, the beautiful Crimson Princess Knight. She leads her party of adventurers into the depths of the dungeon, searching for a legendary treasure with the power to rebuild her fallen kingdom.

While many desire to be by her side, that position is already occupied by Matthew, a former adventurer universally despised as a cowardly weakling and a parasite. The townspeople believe he does nothing but squander Arwin's hard-earned money on alcohol, gambling, and women. However, this is a facade. No one in the city, not even the princess knight herself, knows Matthew's true nature. He is a ruthless protector who does not hesitate to dirty his own hands, working in the shadows to eliminate any threat to Arwin. He is her lifeline, willing to sacrifice everything for her sake, even as he allows the world to see him as worthless scum.

The narrative follows Arwin's dangerous expeditions into the dungeon and Matthew's secret, often brutal, efforts to safeguard her from threats both within the monster-filled labyrinths and the corrupt underworld of the city. A notable early arc involves a monster stampede that leaves Arwin trapped in the deepest section of the dungeon, the Thousand-Year White Nights. Despite bearing a curse that weakens him severely without sunlight, Matthew ventures into the lightless depths to rescue her, facing vicious monsters and the sinister machinations of a growing religious cult, the Sun God's evangelists. As the story progresses, Arwin begins to suffer from a mysterious affliction, and Matthew's past returns to haunt him when a friend presumed dead reappears, dragging them into a conflict over a hidden fortune that involves the city's public authorities, its criminal underworld, and the encroaching religious order.