A marathon turns into a moving crime scene when a shooter begins targeting runners during the race. Jane and Maura, who had planned to compete in the race, instead find themselves racing to catch the killer before another runner goes down. The crowded and impassable city forces Jane and Maura to conduct their investigation on the ground and in secret so the hundreds of thousands of spectators don't become casualties of a mass panic.

When an identity thief with no apparent identity of his own is found brutally murdered with an ice pick, the investigation leads Jane into Boston's Irish-mob past. A startling discovery about the victim sends Maura on a deeply personal quest. Meanwhile, Frankie Jr. tries to help his dad add to his client base.