Liam O'Malley is on his way to a school history trip, to re-enact Cromwell's 1649 Siege of Drogheda. He ends up in 1649 for real - just as Cromwell and his army have breached the walls of the town. With the help of a mysterious girl called Aoife and a horse called Ferdia, he manages to escape the slaughter. Liam and Aoife travel to towns around Ireland to warm people about the dangers of Cromwell and his New Model Army.
Can they reach the towns in time? Can Liam get back to the present day?