Set twelve years after the end of World War II, Wonderful Fool tells the story of Gaston (Gas) Bonaparte, a failed French seminary student and a bona fide descendant of Napoleon himself who decides to forgo formal church endorsement and travel to Japan as a missionary. Having met a native Japanese through correspondence, he embarks upon a spiritual adventure, hoping to use the home of Takamori, a clerk, and his sister, Tomoe, as his “base.”

While reading one of Gas’s poorly written letters in broken Japanese, Takamori realizes with some horror that Gas is coming to visit them. Unimaginative and unsentimental, they harbor no suspicions about his plans to spread, independent of his church, the news of faith and love to the long-neglected Orient. Instead they conjure images of a French nobleman or film star who will honor them with his visit. When Gas arrives in Japan on a third-rate steamer, they find it unusual and unsettling. Upon first acquaintance, both Takamori and Tomoe feel betrayed; a descendant of Napoleon should not be a bumbling, clumsy oaf, “a tramp with the body of a horse.” However well-intentioned, he clearly is utterly ineffectual, unable to speak or understand Japanese except in the most primitive way, and thus completely ill-prepared for cross-cultural communication.

Their trip to a sushi bar on the way home from the shipyard becomes emblematic of the way in which the gangly, uncoordinated Frenchman consistently scandalizes his hosts. Brandishing a Japanese loincloth—given to him by a malevolent sailor on the steamship—as a table napkin, Gas humiliates his hosts. In his simple, trusting manner, Gas later mistakes the advances of a prostitute for the simple congeniality of the Japanese people, marking himself as clearly a stranger in a strange land, a wayfarer whose language and thought processes set him apart from everyone. In a telling moment, End has Takamori observe, “All he’s done since he arrived is walk around and make friends with dogs and children,” moving Gas toward the Christ figure he is clearly intended to become. Tomoe is equally mystified at Gas’s presence, wondering if Gas could be a smuggler or a spy.

After a series of embarrassing episodes, Gas begins to sense that he is imposing on his gracious hosts and announces that he is leaving behind their polite but restrained hospitality, ostensibly “to meet more Japanese people,” but intending privately to embark upon a personal crusade to bring the love of God to the streets of Tokyo. Accompanied only by the mongrel of a dog that has befriended him, Gas moves through the squalor of Tokyo’s underworld as he steadily gropes toward his own destiny—toward his own Gethsemane and eventually his own Golgotha.

The key relationship in the novel emerges in an encounter between Gas and the gangster End soon after he has left the home of Takamori and Tomoe. Having taken up with the lowlifes of urban Tokyo, Gas finds himself in the company of the tubercular End, a professional killer who has in process a plot to murder the man he believes is responsible for his brother’s death. Kidnapping Gas, End tries to compel him to help him get revenge, but Gas repeatedly manifests an innocence and love uncommon in the streets of Tokyo and endears himself to the hardened and morally drained underworld figure. As the story moves to its swift conclusion, Gas thwarts End twice and eventually dies in saving both men from killing each other. His climactic and heroic acts on behalf of two criminals beyond redemption earn for him the reverence from Takamori and Tomoe which his tenderness and tolerance so clearly warranted. In a final scene, Gas, apparently drowned in his mission of mercy, is remembered as a “lone egret, flapping snow-white wings,” a traditional Japanese figure of peace and transfiguration.