The eldest brother in a Boston "Brahmin" family (direct descendents of passengers on the Mayflower) is found in the harbor dead after an apparent sailing accident. But Maura's close connection with the family and Jane's resentment over the wealth they display threaten to derail their investigation. Angela, meanwhile, tries her hand at selling a Polynesian health drink.

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.