After receiving a call from Hong Hee-joo's kidnapper, Paik Sa-eon attempts to learn more about him — only to find it's hard to escape his surveillance.

Sa-eon works quickly to track down the caller, forcing Hee-joo to stay hypervigilant. Despite her fears, she continues to play along with the ruse.

Sa-eon and Hee-joo's relationship shifts after her harrowing rescue. Determined to catch the kidnapper, Sa-eon goes full force — with the help of 406.

As Hee-joo and Sa-eon grow closer, the threat from the kidnapper intensifies. Sa-eon discovers an ominous and shocking clue from a past figure.