Ginta Toramizu is a young boy from Tokyo who struggles with poor eyesight and physical weakness, finding solace only in a recurring dream of a fantasy world where he is strong and heroic. After dreaming of this world 102 times, a mysterious door appears before him in class, offering him the chance to step into the reality of his visions. He arrives in a world known as MAR Heaven, a realm that resembles a living fairy tale, and discovers that in this place, he possesses superhuman strength and perfect vision. He soon learns about ARMs, magical artifacts that can transform into weapons, summon monsters, or grant various other abilities, which are the primary tools of power in this world. Accompanied by a witch named Dorothy, Ginta ventures into a dungeon and finds a unique ARM named Babbo, a ball-and-chain weapon that is alive, can talk, and has lost his memories of his past as the former weapon of a powerful and evil knight named Phantom.

As Ginta explores MAR Heaven, he learns that the world is under threat from the Chess Pieces, an evil army led by the mysterious Phantom, the Queen Diana, and the King Caldia's Orb, who seek to conquer the land and turn it into a nightmare. Ginta befriends a diverse group of warriors, including the monkey-like farmer Jack who fights with a shovel and plant-based powers, the polite but hyperactive princess Snow who wields ice-based ARMs, the cursed and stoic young man Alviss who originally summoned Ginta, and the lazy thief leader Nanashi. Together, they form Team MAR to oppose the Chess Pieces. Their primary conflict unfolds through the War Games, a structured fighting tournament held within the floating Lestava Castle, where Team MAR must battle the Chess Pieces' elite soldiers, known as the Thirteen Zodiac Knights, to save the world.

Throughout the tournament, the narrative focuses on strategic and creative combat, relying less on raw power and more on the imaginative use of ARMs and teamwork. As Ginta grows stronger, he discovers that the King of the Chess Pieces, Caldia's Orb, is possessing the body of his long-lost father, Danna, and that Queen Diana is the older sister of his ally Dorothy. The story culminates in a final battle inside Lestava Castle, where Team MAR defeats the leaders of the Chess Pieces, breaking the curse on Ginta's father and allowing him to return home to Tokyo with his family, while leaving the world of MAR Heaven at peace.