Episode 5 of GG Precinct continues to unravel the mystery surrounding the serial killer, introducing new suspects and shocking revelations. The episode begins with a flashback to 2001, where Shui-yan, a cram teacher, helps a young student named Hsien master his idioms. Hsien's intelligence and dedication impress Shui-yan, who becomes his mentor.
In the present day, Hsien remains evasive during his interrogation, refusing to answer Ming-han's questions. However, Ming-han has a plan. He reveals to Shui-yan that his former student is the killer, hoping to provoke a reaction. Shui-yan initially doubts Ming-han's theory but agrees to talk to Hsien.
During their conversation, Shui-yan is shocked and disgusted to learn that Hsien has been sleeping with his students. Hsien defends his actions, claiming that it's a common practice in the education system. Shui-yan loses his temper and attacks Hsien, revealing that he was not the one who solved his murder idiom.
In the chaos, a drug dealer named Po-lun escapes from the precinct. He teams up with Shao-nien, a former undercover cop, and they go on the run. Meanwhile, Mu-ting is found dead, and the precinct discovers that the killer is a delivery person.
The precinct investigates several suspects, including Cheng-ta, Pai-sheng, and Hsien. However, all of them have alibis. The Commissioner decides to take a risky approach and use himself as bait. He holds a press conference, mentioning the fourth idiom from Shui-yan's book, but intentionally mispronounces it.
Shui-yan insists on visiting Mu-ting's home, believing he might find clues that the cops missed. He sets a trap for the killer, using Tzu-ching as bait. However, the plan backfires when Shui-yan manages to escape and attack the cops.
As the precinct continues their investigation, they receive a tip from Shao-nien. He reveals that Po-lun saw the killer leaving the scene of the first murder. The killer is not a delivery man but a woman. The precinct sets up a trap, waiting for the killer to arrive.
The episode concludes with a cliffhanger as a delivery bike speeds towards the precinct. Chang prepares to confront the killer, but the identity of the mysterious woman remains a mystery.