Irish revolutionaries. British spies. German mercenaries. And a submarine that shouldn't exist. The year is 1921, and a shaky truce has been established in Ireland's War of Independence. As negotiations begin in London to finally free the Irish from British control, a young intelligence agent named Esther Sinclair discovers that a nightmarish weapon has gone missing in France, and may have been smuggled over to Northern Ireland. Two teenagers, Akiko Regan and her cousin Liam O'Leary, have made their own discovery - that Akiko's father, Michael, is the leader of the mysterious rebel group known as the Selkies. Michael is an engineer who trained under the famous John Philip Holland in New Jersey. At the start of the century, Holland's designs were used to establish the submarine fleets of the United States, Great Britain, Japan, the Netherlands and Russia. But his very first submarines had been built for the Irish republican movement. Hardly anyone knows that a Holland vessel has been operating in the Irish Sea, right under the nose of the Royal Navy. Michael has kept this part of his life secret, trying to keep Akko and Liam out of the violence, but now the family find themselves drawn into a crisis that threatens not just their lives, but the future of their country.