Harry Langford helps Five-0 investigate a series of murders that follow the plot of a fabled unpublished crime novel from the 1920s; Tani and Junior's friendship takes a meaningful new turn.
Thony struggles to make ends meet as an undocumented worker after coming to the US in search of medical treatment for her son. After witnessing a murder, Thony cleans the crime scene and begins living a double life.
Thony attempts to sever ties with the crime syndicate when the FBI comes asking questions, but when her son's health declines, she ends up deeper involved with the syndicate.
Thony is given a dire ultimatum by Garrett that could compromise her son's access to medical treatment; a rift forms between Fiona and her son, Chris, when a secret is revealed that has major consequences for his future.
Thony desperately takes matters into her own hands when she faces a setback with Luca's treatment; Garrett makes a risky demand of Thony, pushing her deeper into moral ambiguity.