On a rainy night in Busan, So-young (Lee 'IU' Ji-eun) leaves her baby outside a 'baby box', a safe place set up in Korean churches for new mothers to leave unwanted infants. Instead, he's picked up by Sang-hyun (Song Kang-ho) who runs an unofficial adoption brokerage and plans to find him a new home. So-young tracks down both Sang-hyun and his business partner (Gang Dong-won) and decides to join their pursuit - unaware they're being tailed by two detectives (Doona Bae, Lee Joo-young) who are determined to stop them.