Coming from Episode 10, this episode starts with Mayor Choi Doha rushing to the hospital after his wife's car accident. Thankfully, Director Juhee wasn't seriously hurt but it seems that Gong Jihoon has something to do about her accident.
As for Miho, it turns out that her secret contact was no other than Jang Hyejin. She gave her the blood sample in the previous episode and Hyejin got it tested in the U.S.
Miho then received a call from Hyejin saying that a drug was detected in the blood sample. It's a new drug which hasn't been reported yet in Korea.
Knowing that there could be an illegal drug substance in the medications given to the inmates, Miho then refused to administer it on one of the patients. Instead, she just put him to sleep and gave him some painkillers.
Miho then reported her new findings to Changho.
During their conversation, the two talked about how only the "model inmates" are getting affected, and how some of them got leukemia.
Jang Hyejin is back in Korea, and Miho went to pick her up at the airport.
Miho dropped her at a safe house but little did they now that Hyejin's husband, Jaeho, was able to follow them. Later on, this resulted to an unfortunate incident.
Mayor Choi Doha also arrived at the scene, and there he found out what Miho and Changho have been doing by reading the text messages from Hyejin's phone.
Without knowing that Mayor Choi Doha has already ordered Gan Soochul to kill her and Changho, she continued getting blood samples from the other inmates.
Meanwhile, Changho, who has been busy taking care of the matters inside the prison, set another meeting with "Big Mouse" through Park Yoongap.
He didn't know that a riot was already ensuing inside the prison as he was down at the special ward waiting for "Big Mouse".
The lights are out, and someone sat across Changho. When he lit up his lighter, Changho saw their Room Leader, Noh Park. Is he the "Big Mouse" we have all been eager to meet? We'll soon find out on Big Mouth's 12th episode.