TWICE, a group under Seoul's Secret Service (SSS or S3) led by Jihyo, was sent to an operation to save the laburis. The operation was a success and TWICE was disbanded afterward. Years after the incident, a series of events led the members back to each other. A terrorist attack led to a member's disappearance. Jihyo, the only active member left from TWICE to serve the SSS, was tasked to find the missing member. However, it seems like the task is too daunting for her to handle.
A crucial argument between the members had caused Jeongyeon to leave, only to lead into a disastrous accident leaving everyone shocked, panicked and worried.
Jeongyeon loses her memory after the accident, bringing her 10 years back, before she joined JYP Entertainment. The members are tasked to bring her memory back, and to apologize for what they did that lead to this unwanted situation.