In "Astarotte's Toy," the young succubus princess Astarotte, called "Lotte" Ygvar, needs the life essence of men to sustain herself, restore her magic, and secure her royal bloodline's continuation. Succubi traditionally keep a harem of men for this purpose, but Lotte hates males, which puts her survival at risk as she nears adulthood. To help her get past this aversion, her aide Judith Snorrevik travels to the human world to find a man Lotte might accept.
Judith returns with Naoya Touhara, a 23-year-old single father who reluctantly leaves his daughter Asuha in the human world. As the first member of Lotte's harem, Naoya adjusts quickly to his new surroundings, focusing on pleasing the princess without any romantic feelings. However, when Asuha is allowed to join them in the magical realm, Naoya's relationship with Lotte becomes strained. Still, he keeps working to mend their bond.
Naoya’s presence in the magical realm turns out to be more than a coincidence, and as his connection with Lotte deepens, the threads of fate intertwine, affecting everyone in this mystical world.