During World War I, a Scottish soldier named Private Charles Plumpick is tasked with disabling a bomb planted by retreating German forces in a small French town. Upon his arrival, he finds the town deserted except for the inhabitants of the local insane asylum, who have all escaped. Mistaking him for a figure of authority, they crown him the "King of Hearts" and continue their delirious celebrations. Amidst this chaos, Charles navigates through their whimsical world, forming a bond with a beautiful but naive young woman and realizing the utter absurdity of the situation around him.

As Charles interacts with the asylum residents, he grapples with the profound difference between their seemingly innocent madness and the brutal reality of war. He faces moral dilemmas that compel him to reconsider his sense of duty and loyalty. Throughout his stay, the line between sanity and insanity fades, challenging his own preconceptions about what constitutes true madness. All the while, the ticking bomb remains a silent yet ominous backdrop to the unfolding drama, pushing Charles toward a moment of truth that will define his courage and humanity.