Mac and Jo find victim Rita Lowman's charred remains from being trapped inside her apartment building's elevator as the overhead pipes ignited. Detective Flack relays that the security camera picked up recently paroled arsonist Leonard Brooks (seen in #901) moonlighting as an elevator maintenance worker. They head to his apartment where Mac receives a call on the landline from Leonard hinting that there is more to come. During the autopsy, Dr. Hawkes notices a pendant with the initials H.B. on the body, which matches Flack's discovery that the name Rita Lowman was an alias. A second victim, Jimmy Conklin, dies eating his lunch and Lindsay concludes that the spiked mayonnaise in his sandwich sparked a chemical reaction after making contact with the water he ingested shortly afterwards. The sandwich wrapper leads them to the deli owner who informs them that he hired Leonard to work the counter, which gave him the ability to tamper with Conklin's food. Meanwhile, Detective Jamie Lovato leaves her undercover assignment and transfers from Narcotics to Homicide to join the team. Lovato uncovers that Rita's real name is Jennifer Brooks, meaning she is Leonard's birth mother, and that Conklin was her foster son. Jo analyzes Leonard's prison therapy tape recordings to learn that following the death of his father, Jennifer took Conklin under foster care and that Leonard suffered from the same nightmare related to the story of Saint Aidan. Danny alerts the team to the existence of Jennifer's other foster child, Rachel Moore, who was recently kidnapped by Leonard after he created a diversion at the rehab center she was staying at. Given the information that Jennifer was a burn victim nurse, Mac deduces that Leonard likely took Rachel to Saint Aidan's, a now abandoned hospital Jennifer worked at previously. The team finds Leonard preparing to set Rachel on fire while he angrily questions why she didn't rescue him from the physical abuse inflicted by their mother when they were children. Flack takes him down when he is caught off guard by Mac and Leonard later reveals his resentment towards his mother for beating him while caring deeply for her burn patients.