A true-crime enthusiast, Dae-Man (Kwon Sang-Woo) and a Sr. Detective Noh (Sung Dong-Il) now demoted under his Jr. launches a joint murder investigation to help a mutual friend and detective, Joon-Soo (Park Hae-Joon) who is wrongfully charged with murder. The duo team finds out a series of murders taken place are related to the case and unraveling them they find the murders are done by 3-4 members to avenge their cheating wives. They were done by swapping targets ("exchange" murders) to avoid being a suspect and creating an airtight alibi.