On the brink of the First World War, Albert's beloved horse Joey is sold to the Cavalry by his father. Against the backdrop of the Great War, Joey begins an odyssey full of danger, joy, and sorrow, and he transforms everyone he meets along the way. Meanwhile, Albert, unable to forget his equine friend, searches the battlefields of France to find Joey and bring him home.

When an attack on the Kingsman headquarters takes place and a new villain rises, Eggsy and Merlin are forced to work together with the American agency known as the Statesman to save the world.