Ryung officially becomes a member of the Wangsoori Subway Police and Ji Seok's partner. Ryung gets into trouble from the start, because she sneaks into the tunnels. Meanwhile, an elderly man who delivers packages via subway becomes the victim of an assault. Ji Seok realizes that the package is related to criminal activities. While Ji Seok wants to hand the case over, Ryung acts before she thinks, as always.

Ryung and Ji Seok save Choi Do Chul just in time. Throughout their investigation, Ji Seok miraculously learns to appreciate Ryung's over-the-top nature. Meanwhile, a star professional fighter becomes the PR Ambassador for the Subway Police. By chance, Ryung comes across his dirty secret and the Wangsoori Subway Police finally get a lead on the Grasshoppers.