After three men are killed by garrotte strangulation, the BAU notes the UnSub's pattern of stealing his victims' wallets and cutting pieces of hair off each of them. The UnSub strikes again by strangling a teenage boy playing a video game late at night with his friend. Recovering the phone of one of the initial victims helps them locate the number of a young woman, Ashley, who the victim spent the night with before he was killed. Before they can question her, she is found dead in an alley as the UnSub was forced to kill her when she stumbled upon his lair, unbeknownst to the team. The match of Ashley's address to the teenage boy's friend leads to the discovery that the common thread among the victims is their interaction with people who all lived in the same building. The UnSub targeted them for being bad influences and perceived himself as a protector.When they investigate the apartment building to find evidence pointing to the night attendant, Tanner, they learn he had a son who was inadvertently killed in a chokehold while roughhousing with a friend. His momentary negligence led to extreme guilt and the garrotte method he adopted was to mimic the way his son died. Tanner was using pictures from the victims' wallets and their hair to create a scrapbook documenting the people in the apartment building he considered to be his new family. The team tracks down Tanner at the home of his pregnant ex-wife who left him after their son died. She goes into labor and complications arise, leaving Tanner emotionally vulnerable enough for the team to apprehend him. Reid is able to deliver the baby successfully and the team celebrates their victory at Rossi's favorite karaoke bar before it is closed down.