Ambrose and John Clark are back in their second Revolutionary War adventure. Fresh from delivering a top-secret message to George Washington in Patriots, Redcoats, and Spies (2015), the 14-year-old twins have now been recruited to man the American Turtle, the world's first submersible used in combat, in order to attach explosives to the hull of a British ship. Things are complicated by the fact that Loyalist spies are reporting their every move, and their older brother, Berty, is captured and scheduled to be hanged as a traitor. The story moves at a brisk pace with plenty of musketry and bloodshed, and the technical information about the Turtle's creation is interesting rather than dull; readers hearing of the submarine for the first time will be intrigued. Discussion questions, a list of the historical characters who appear in the series, a glossary, and reproductions of historical letters support the story and make this a novel that could easily be incorporated into a classroom or homeschool lesson.