The protagonist, Masayoshi Tachihara, a severely overweight and bullied high school student, attempts suicide by hanging in his apartment. However, a sudden, powerful earthquake causes the ceiling beam to break, saving him from death but leaving him with a large hole in his ceiling. Peering through the hole, he discovers a black, liquid-like surface that, upon investigation, leads to another world. Driven by curiosity, he climbs through and finds himself in a brick room with a circular portal.
He enters a larger room containing a mummified corpse and a letter. Upon picking up the letter, he inexplicably gains the "Language Comprehension Skill," allowing him to read it. The letter is from Hardy Urnus, a retired Archmage, who explains that he created a magic to connect worlds but couldn't cross himself. He reveals that the protagonist is from another world and, as a "gift," Hardy has initiated a mandatory "Inheritance Magic" that will transfer his skills, magic, and levels to Masayoshi. This process is agonizing and takes ten hours.
Masayoshi endures immense pain as he inherits Hardy's abilities, including skills like "Presence Perception," "Stealth," "Body Maintenance," and "Suppression Skill," along with various magic and increased physical capabilities. He also discovers he has become significantly thinner and more muscular, transforming into an attractive young man.
Upon waking, Masayoshi tests his new powers. He finds he can use magic, such as "Fireball," and possesses immense physical strength, which he learns to control with the Suppression Skill. He also discovers a "Body Maintenance Skill" that optimized his physique. He then ventures into the world beyond Hardy's house, which is a forest. He encounters and easily defeats a powerful Orc using an advanced lightning magic called "Lightning Caliber," realizing the immense power he now wields. He also learns that his original world's magic is usable there.
Returning to his own world, Masayoshi finds his life drastically changed. He confronts his bullies, including the school president, Ryu Tsuyuara, and his associates, using his newfound powers and skills to intimidate them into leaving him alone. He also uses his abilities to help a girl injured in a traffic accident.
Masayoshi begins a double life, attending school while exploring the other world during his free time. He buries Hardy's remains and begins to study the Archmage's books, which contain information on magic and equipment. He encounters more monsters in the forest, including a powerful slime and a goblin, which he defeats using his combat skills and magic. He also begins to reconnect with his former friends, who are surprised by his transformation and apologize for their past inaction. Masayoshi's life, once filled with despair and bullying, is now filled with the potential for adventure and self-discovery in two worlds.
