A dark secret haunts students at the prestigious Sinha High School. Between the school day’s third and fourth periods lies a little-known “3.5th period!” This period is unlike any other, as a place where time moves 10 times slower than usual… and 10 minutes becomes 100 minutes. When Baek Ah Jin (ZOA) briefly finds herself in this perilous period, she meets a male student named Kang Da Won (Lim Ji Sub). Da Won is permanently trapped in Period 3.5, and Ah Jin resolves to help him escape. To do so, Ah Jin will have to uncover the mysterious secrets of Period 3.5, but she soon discovers that larger forces in the school want to keep its existence under wraps. Can Ah Jin learn the truth – and help free Da Won?