Tom is only 13, but after his father is killed while fighting for the Union, he must take on the responsibility of someone much older. He signs up for work at the Brooklyn Navy yard where men are busy building battleships for the Civil War. But instead, Tom is sent to a bustling warehouse in another part of the city. There, amidst the roar of furnaces and clanging iron, 100? men are frantically pounding together the strangest ship Tom has ever seen.

Because he is literate and quick on his feet, Tom becomes assistant to the ship’s inventor, a gruff, boastful man named Captain John Ericsson. He spends his days running mysterious errands for the captain and relaying messages throughout the warehouse, and he soon learns that the Union army has very grave plans for this iron ship… and that Ericsson is the only one who believes it will float.

Meanwhile, Tom is thrown into a grave dilemma of his own. His position as warehouse messenger has made him a target of Confederate spies who offer him money for secrets about the ship… not sure where the plot goes from here.