The story begins with the protagonist, Fujimiya Soujirou, experiencing a fatal wound after a confrontation. He reflects on his past, detailing his upbringing in the prestigious Fujimiya family. His mother, an only daughter, lived in her strict grandfather's traditional Japanese mansion. His father, treated poorly by the grandfather, eventually left the family when Soujirou was in middle school. Soujirou himself refused to leave the mansion, not out of dislike for his father or fear of poverty, but due to his deep fascination with the family's antique sword collection, particularly a large tachi gifted to him by his grandfather. He dedicated himself to sword maintenance and practice, becoming exceptionally skilled.
The narrative then shifts to the night of the attack. A security system in the family's vault is triggered, alerting guards. Robbers, masked and armed, are discovered inside. Soujirou, alerted by the commotion, witnesses the brutal murder of the guards and his grandfather by a gunman. Driven by an unusual detachment towards the attackers, whom he views as less than human, Soujirou intervenes. He disarms the gunman by severing his wrist and then kills him. However, he is then fatally stabbed from behind by another robber, revealed to be his own father, who had joined the thieves.
Soujirou finds himself in a luminous space, where he encounters a divine being and his beloved tachi, Hotarumaru. He learns that his world is doomed and he is offered a chance to be reborn in another world. He accepts, bringing Hotarumaru and a kodachi named Sakura with him. He is given basic skills: language, reading comprehension, and simple appraisal.
Upon arriving in the new world, Soujirou, dressed in his school uniform (now modified into a delinquent-style outfit), finds himself in a grassy plain with Hotarumaru and Sakura. He soon meets Sistina, a "Saint Priestess" with unique abilities, who was attacked by bandits. Soujirou, with Hotarumaru's guidance, swiftly defeats the bandits. Sistina, impressed by his combat prowess despite his lack of combat skills and his unusual "Magic Swordsman" class, proposes a contract. Soujirou, recognizing her kindness and the potential for a fulfilling life, agrees. He modifies the contract to allow Sistina some autonomy, and they form a bond.
Their journey begins towards Mikania, a city with a teleportation array to a "Tower." They learn that Towers are sources of monsters and that Priestesses like Sistina are obligated to explore them in exchange for benefits like tax exemption. They sell the bandits' weapons and the pelts and fangs of a "Grass Wolf" they encountered, earning a substantial amount of money. They also acquire a horse-drawn carriage and a creature called a "Rama."
Upon reaching Mikania, they discover that due to Sistina's status as a contracted Priestess, they are exempt from entry fees but are obligated to explore a Tower. They check into an inn, and Soujirou, along with Hotarumaru and Sakura, begins to adapt to this new world. Hotarumaru, now in human form, offers to train Soujirou in swordsmanship, while Sistina, with her extensive knowledge and skills, becomes his guide and companion. The narrative ends with them preparing to enter the Tower, a place where monsters endlessly spawn, hinting at the challenges and adventures to come.
