Saito Masato only has eyes for his childhood friend, which makes him the sole boy at school immune to the charms of the new transfer student, Tsukishiro Saya. Frustrated by his indifference, Saya finds herself in an unexpected situation with Masato, leading to a tumble down the stairs. When Masato regains consciousness, he discovers he has woken up inside Saya's body. The truth soon emerges: Saya is actually a succubus, and the two are now stuck in each other's bodies until they can figure out how to swap back.

The story follows Masato as he navigates life in a girl's body, particularly one belonging to a supernatural being. He quickly learns that Saya possesses a powerful allure that makes boys eagerly do her bidding. As Masato experiences the pleasure and social power that comes with Saya's form, he begins to question whether he truly wants to return to his own body. The narrative explores his internal conflict between his lingering feelings for his childhood friend and the intoxicating advantages of his new situation.

The setting is primarily a modern Japanese high school where supernatural elements like succubi exist unnoticed alongside everyday student life. Kimiiro Days is part of a larger tankoubon collection that includes five additional body-swapping stories, expanding the concept into various fantasy scenarios. These include tales involving cosplayers, shrine maidens, elves and orcs, and nurses, each putting a unique spin on the theme of characters experiencing life in another person's body.

Another story within the collection, Possession Power, shifts focus to a group of boys planning a maid cafe for the school culture festival. Outnumbered by one girl, the boys face opposition from the class president. A student named Komatsu devises a solution using a special drink that allows him to transfer his soul into someone else's body for a limited time. By possessing female classmates and a teacher, the boys attempt to use feminine charm to sway the president's vote in their favor.