At eight years old, Kūya Aragaki is caught in an accident at sea and inexplicably transported to another world. At first, the strange new world appears peaceful, but Kūya soon discovers that humanity is facing extinction. For reasons nobody fully understands, the world's men eventually transform into terrifying monsters known as Renma. Even the master who rescued and raised the young Kūya eventually succumbs to the same fate, leaving the boy alone. Years later, Kūya has become the last remaining man. Then a boat washes ashore carrying Riruru, a girl from a neighboring continent where only women remain. Her arrival reveals just how desperate humanity's situation has become: without men, the human race has no future. Now possessing both extraordinary fighting abilities and something even rarer, his very existence as a human male, Kūya is drawn into a struggle to save a dying world. Together with Riruru and the women he meets on his journey, he must battle the Renma, uncover the mystery behind the disappearance of men, and perhaps even ensure humanity's survival in a rather more intimate way. An action-heavy isekai survival fantasy about the last man alive in a world populated almost entirely by women.