Ren Homura has just moved back to his hometown of Kaminohara, but he is not able to remember much about the time he spent there other than vague flashbacks. When he retrieves his cell phone which he had sent to a shop for repairs, he finds that a mysterious game titled "Monster Strike" has been installed on it. Unaware of its purpose or how it got there, Ren decides to open it, an action that will change his life forever as he finds himself suddenly attacked by a strange man. Now trapped on the roof of his school which strangely has been designated as a "stage," Ren is shocked to see the man summon a realistic monster from the game, one that is more than capable of inflicting damage on a human. His only chance of getting out of this situation alive is by summoning the dragon Oragon, who tells Ren that he will crush the opposing monster for him. Unfortunately, it turns out that Oragon is a tiny dragon that is completely useless!

When Light is kicked out of the Concord of the Tribes, his former comrades instantly turn on him. Light escapes this diabolical act of betrayal by the skin of his teeth...only to find himself in the deepest part of the Abyss, the most dangerous dungeon in the realm! To avoid being eaten by carnivorous monsters, he uses the Unlimited Gacha, his sole magical skill. But where it previously only produced junk items, this time Mei—a gorgeous Level 9999 fighter in a maid outfit—springs forth! Fast forward three years and Light has carved out his own kingdom in this backwater dungeon, summoning more beautiful Level 9999 warriors who swear absolute fealty to him. Now a powerful Level 9999 Overlord himself, Light plans to ascend to the surface and take revenge on his betrayers one by one!

Ever since childhood, Scarlet El Vandimion has preferred to deal with wrongdoers by using her fists. However, to preserve her family's image, she suppresses her instincts and tolerates the abuse inflicted by her childish fiancé, Second Prince Kyle Von Pallistan. Only Kyle's brother, First Prince Julius, is aware that there is more to Scarlet than meets the eye, but she cares little about his growing interest. In the middle of a ball, Kyle accuses Scarlet of harassing Terenezza Hopkins, a commoner that has taken his heart. Scarlet's engagement to Kyle is nullified, leaving her outraged that all of her efforts to endure thus far have amounted to nothing. Scarlet dons her studded leather gloves, once again becoming the "Mad Dog Princess." Joined by Julius and werebeast Nanaka, Scarlet is ready to give filthy nobles a taste of their own medicine—one punch at a time.

Follow Liko and Roy as they unravel the mysteries that surround them and encounter Friede, Captain Pikachu, Amethio, and others during their exciting adventures!