Secret D-Day summary: Yoo Chae-rin is a woman rumored to change men every single day. Right before sleeping with her latest boyfriend, she dumps him immediately upon seeing his 'disappointing size'. Feeling disillusioned about dating, she is carrying coffee in a cafe when she bumps hard into someone and spills it. Looking up, Chae-rin expects an apology, only to see an unexpectedly tall and handsome man. However, instead of apologizing, the man coldly brushes off Chae-rin's hand and vanishes. After this terrible first meeting, Chae-rin actually finds herself strangely intrigued by him. After asking around, she discovers he is a peer in her year named Jang Ryu-hyeon... Although handsome, he is notorious for his terribly rude personality and is dubbed the 'campus representative loser.' Chae-rin formulates a plan to thoroughly seduce him, make him fall head over heels, and then mercilessly dump him. In a relationship that starts with an unexpected twist, will he remain Chae-rin’s plaything until the end? +