Succubi are a race of beings who survive by having a harem of men from which they can suck 'life essence' from. However, Astarotte Ygvar, the crown princess, hates men. Under relentless pressure from Judit, her chief of staff, to grow up and start building her harem, she agrees to start but only with a human male. In the Creature World, humans are considered mythical creatures, with none having been seen in a thousand years. To her dismay, Judit manages to travel to the human realm and return with a 23 year old human male named Naoya Tōhara, who accepts the offer to found Astarotte's harem.

A festival celebrating the World Tree is taking place and Astarotte's friends help her prepare for a singing performance. After some training and a bath, the girls, along with Naoya, head to the festival, where they have a brief run in with Sigurð. With Astarotte still nervous when it's time for her performance, Asuha helps her out by joining in the singing. After the festival, Asuha brings up the question of what Naoya intends to do after summer vacation ends.

Astarotte and Naoya have their date at the amusement park where she finally manages to convey her feelings to him. The two promise to meet again while Mercelida pays a visit to Asuha. Some time after Naoya and Asuha return to the human realm, Ini makes use of Naoya's cellphone and the World Tree sapling at their house to establish communications with them, and the duo learns that the clover-shaped mark on Rotte's back was actually a bug bite and Judit gets some "punishment" for her mistake. Soon after Ini manages to stabilize the World Tree's magic to create a proper portal between the worlds, Naoya and Asuha use it to return to the monster realm and rejoin Lotte, whose growth marks finally appeared on her back.