By day, London's Steam Emporium is a bustling Christmas market, an all-ages toyshop full of technological marvels. By night, its volunteers assemble—a ragtag community of wounded servicemen and women—under the tallest Christmas tree in England. They are the empire's forgotten heroes, fallen on hard times.

But this year they have an exciting addition: old Red Mulqueen, newly arrived from abroad, has fighting spirit and the most extraordinary clockwork leg. He is here on a mysterious mission, and is wanted by the Leviacrum, Britain's tyrannical authority.

Red's plans take a turn, however, when he saves the life of a troubled young schoolboy, Edmond Reardon, and becomes the object of the lad's incorrigible curiosity. They strike up an unexpected friendship. But when danger descends on the Emporium, Red, Edmond and their veteran friends must band together to fight for what matters most during Christmas—each other.