After a strange incident, the Phantom Thieves wander into a bizarre realm where its citizens are living under tyrannical oppression. Surrounded by a military group named Legionnaires, they find themselves in grave danger until a mysterious revolutionary named Erina rescues them and offers an enticing deal in exchange for their help. What truth lies behind Erina and the deal she offers to the Phantom Thieves.

Sometime after their triumph as the Phantom Thieves of Hearts, the group gathers at Cafe LeBlanc while preparing for graduation, their ordinary lives seemingly restored. Their calm is shattered when they are abruptly pulled into a bizarre alternate reality, a medieval European-style realm known as a Kingdom, ruled with an iron fist by the ruthless Tyrant Marie. With frightening ease, she overwhelms most of the thieves and bends them to her will through brainwashing. Just as Joker and Morgana are on the verge of falling to her power as well, they are saved by Erina, the fierce leader of a band of freedom fighters called the Rebel Corps. Vowing to help Erina overthrow the tyranny that grips this strange world, the remaining thieves join her cause and set out to spark a revolution.