Just days before Christmas, college sophomore Hasegawa Yuuta gets the worst kind of holiday surprise: his girlfriend's been cheating on him. Depressed and dodging couples on every corner, he stumbles into Shinohara Mayu—an irresistibly bold underclassman dressed like Santa and full of mischief. She's sweet, she's sassy, and she's suddenly everywhere Yuuta goes, delighting in teasing him back to life. But is this just playful fun... or the beginning of something more?