Chae-yoon, a top-notch police crisis negotiator, is summoned to a situation where her boss has been taken hostage. Observing through the control room’s monitor, she encounters the enigmatic and ruthless captor, Tae-gu, whose intentions are hard to decipher. With a 21-hour ultimatum imposed by Tae-gu, Chae-yoon relentlessly attempts to outwit the unnervingly composed criminal through a series of video calls. As time ticks away, a startling revelation starts to emerge.