The only thing second-year university student Gong Siru knows is that there was an incident last night. The man she kissed is nothing but a silhouette in her hungover mind, and her only clue is a smudge of blood left on her lips. When Siru sees her best friend, Seol Gibaek, with an injured lip, a realization hits her. On top of confessing her long-standing feelings for him in her drunken state, she must have kissed him too.

While Siru wishes to run from Gibaek's rejection and hide in embarrassment, her friends fill her in little by little on the events of the previous night. Just as she assumes that she has pieced everything together, another person's bruised lip catches her attention. Handsome and aloof, Jo Cheong is somewhat of a closed book—most of all to Siru, as he suddenly reveals that her mystery kiss was none other than him.