Nantucket Penny (Henry Kennis Series, Book #6) by Axelrod, Steven - 2020

When people begin disappearing from the island, Nantucket Police Chief Henry Kennis is baffled. The victims are not random -- they're all Nantucket High School alumni. And the only clues left behind are pennies dropped at the sites of the kidnappings. There's an old island tradition of tossing a penny from the ferry as you depart to ensure your eventual return..... Has someone come back to the island with a sinister grudge to settle?

Sippy Bascomb and Todd Fraker were childhood best friends, bonded as fellow victims of bullying at the hands of their classmates. The two men hadn't seen each other in years, but when Sippy comes across Todd's blog airing grievances from the past, the two reconnect and hatch a plan to return to the island. Both seek revenge, but Todd wants their tormentors to face a tribunal and appoints himself judge, jury, and executioner.

When Henry's fiancee joins the missing, he enlists the aid of former intelligence agent Mitch Stone, a native who has returned to Nantucket hoping for a peaceful retirement. The two men must find where their friends are being put on trial for their childhood crimes -- before their deadly sentences can be carried out.