I actually read the manga adaptation first before hunting down the light novel. It's fundamentally a school romantic comedy centered around Asahi Kurobe, a guy who prides himself on being unflappable, and Karin Tachibana, the seemingly perfect and popular girl in his class. The 'main plot' kickstarts when Kurobe sees Tachibana make this incredibly goofy, unattractive face while she's alone, thinking no one is watching. He's the only witness.
Instead of the usual blackmail or embarrassment trope, he finds it weirdly endearing and can't get it out of his head. The story then follows his internal struggle and their awkward interactions as he grapples with this new, hidden side of her. It's less about grand dramatic arcs and more about the slow, meticulous observation of two people circling each other, with this one bizarre, intimate secret between them. The charm is in the hyper-specific internal monologues and the painfully realistic hesitation.
Honestly, the plot summary makes it sound simple, but the execution is in those tiny moments—a shared glance, a stumble over words, the quiet panic when she almost catches him thinking about that face. It's a masterclass in stretching a single, fragile premise across a whole narrative without it snapping.