An undercover ATF agent who infiltrated a gunrunning gang is found dead. At the victim's house, Chin discovers a key that the he traces to a cash filled locker. We initially believe he was stealing from the gang only to discover the money was part of a set up to discover who at the ATF was leaking information. The team nabs the transcriber of the victim's notes and her boyfriend in the crimes. Meanwhile, Mary has adopted a baby but when she gets sick, she enlists McGarrett to help take care of the baby with the help of Chin and Danny. Catherine questions Kuroda Hideaki about Sato's whereabouts so she can help Kono find Adam and becomes a permanent Hawaii Five-0 member.