In the late 1960s, C’est Si Bon is the music bar where every acoustic band’s dream lies. There Geun-tae, a naïve country boy, meets the young musical prodigies Hyung-joo and Chang-sik, and forms the band named after it — the C’est Si Bon Trio. As the three young artists bicker over their music, beautiful socialite Ja-young enters the picture and becomes their muse, and a series of moving love songs come from it. Geun-tae’s pure-heartedness wins Ja-young over but when she accepts a once-in-lifetime opportunity for a shot at an acting career, they part ways. After 20 years, the untold story of their love, song, and youth at C’est Si Bon is finally brought to light.

Ten years have passed since the discovery of inter-cosmic travel. The existence of three parallel worlds have been confirmed. What happens in one cosmos, even the slightest incident, can create a “butterfly effect” that influences events in the other worlds. In response to these discoveries, the “Dimension Police” and “Space Guardian Academy” are formed. The beautiful female investigator Ai is the Space Guardian Academy’s top agent. Along with her trusted android partner, Bridge, she has been assigned to eliminate the inter-cosmic terrorist group DOUBT. Amidst the struggle, Ai finds herself challenged like she never has been before when allies become enemies, enemies become allies, and the mastermind behind the evil plot is revealed.