A sword-obsessed high school student named Fujinomiya Soujirou is killed during a home invasion when robbers break into his house. Just before dying, he cuts down the intruders with his beloved greatsword Hotomaru, but is stabbed in the back by a wakizashi held by one of the assailants. A mysterious being calling itself God offers him a second chance and transports his consciousness together with both blades to a fantasy world. In this new realm, Soujirou discovers that the greatsword is no ordinary weapon: it is Hotomaru, a legendary Japanese national treasure thought lost during World War II, and it can speak. The smaller blade is still too young to have a voice, so Soujirou names it Sakura.

Upon arrival, Soujirou is granted the job title of cursed sword master, or makenshi, which gives him the unique ability to nurture and eventually personify weapons into humanoid forms. His swords can transform into beautiful women, and the deeper his bond with them, the more powerful they become. Soon after entering the world, he encounters a carriage under attack by bandits and rescues a girl named Sistina, a sixteen-year-old acolyte known as a shrine maiden or servant priestess. She has been trained in combat, healing, and negotiation, and she proposes a contract of absolute obedience to Soujirou, binding herself to him as his companion and partner. Together with the personified forms of Hotomaru and Sakura, the group begins to explore the world.

The setting is a fantasy realm dotted with towering dungeon-like structures simply called towers. These towers are filled with monsters and valuable resources, and adventurers descend into them to hunt for food, money, and equipment. The world operates on a job-based system, and the cursed sword master class is rare. Soujirou and his party spend much of their time clearing floors of these towers, battling increasingly dangerous creatures while earning a living and strengthening their weapons.

The early narrative follows Soujirou, Sistina, Hotomaru, and Sakura as they adjust to life in the new world, take on quests, and develop their combat abilities. A major early arc involves a tower anomaly triggered by aggressive monster hunting, which traps civilians inside and forces Soujirou to confront a powerful floor boss known as the Dragomantis. During this battle, Sakura finally awakens and manifests her humanoid form, turning the tide. After defeating the boss and preventing a collapse event called a floor drop, the party is rewarded by the local lord with a significant sum of money, which they use to purchase a large, reputedly haunted mansion complete with a hot spring.

Later arcs expand the cast as Soujirou successfully summons a third cursed sword, and the group travels to a seaside town to repair equipment, leading to beach outings and further battles. New threats emerge, including a notorious bandit group called the Red Meteor, which forces the party to fortify their home and prepare for a direct assault. As the story progresses, Soujirou takes on a larger role in the community by organizing a guild to support other tower explorers, moving from personal survival toward leadership and world-building.

The main characters include Fujinomiya Soujirou, who harbors a deep hatred for evil people and a near-obsessive love for swords, giving him a cold and ruthless streak when dealing with criminals. Hotomaru is an ancient, centuries-old blade who becomes a beautiful woman and develops a deep affection for Soujirou. Sakura is the younger sword who killed him, initially mute but later awakens as a fierce and loyal fighter. Sistina is a diligent and capable shrine maiden who manages the party's daily life and supports Soujirou unconditionally. The story combines dungeon crawling and combat sequences with harem-building and intimate character relationships, framed by the mechanics of weapon growth and personification unique to the cursed sword master class.