Townsend Harris is sent by President Pierce to Japan to serve as the first U.S. Consul-General to that country. Harris discovers enormous hostility to foreigners, as well as the love of a young geisha.

In 1856, an American ship carrying Townsend Harris, the U.S. Consul General to Japan, and his interpreter, Henry Heusken, nears the Japanese seaport of Shimoda. When Baron Tamura, the governor of the province, denies the ship permission to land, Harris asserts that he has come in accordance with the treaty signed between his country and Japan. Believing that all the disasters visited upon Japan since the signing of the treaty have been warning signs from the Gods, Tamura refuses to recognize the pact, but Harris defies him and comes ashore anyway. Although Tamura rejects Harris' status as American Consul, he allows him to remain as a private citizen and grants him the use of a dilapidated old house. When Harris hoists the American flag above his quarters, Tamura orders it lowered and instructs the villagers to ostracize the Americans. In protest, Harris presents Tamura a letter addressed to the Shogun, asking him to confirm his position as American Consul. The Imperial Court in Edo is in a state of indecision over the treaty, and Tamura is instructed to keep Harris happy until a resolution can be reached. Following orders, Tamura invites Harris to dinner. During the course of the evening, Tamura notices Harris' interest in a comely geisha named Okichi, and so sends her to the American to keep him occupied. At first frightened and puzzled by Harris' strange American customs, Okichi is soon won over by his gentleness and compassion. Harris is appalled when he learns that Okichi was sold into a geisha house as a young girl by her poverty-stricken family. Harris' friendship earns Okichi the enmity of the village women, who deem her a concubine and shun her. One day, another ship sails into the harbor, and when Harris rows out to greet it, the captain warns him to stay away because the crew is rife with cholera. When several of the infected sailors dive overboard and swim ashore, Harris warns the villagers to stay away from them, but to no avail. Soon, cholera sweeps through the village and Okichi is stricken. As the villagers perform rituals to cast out the disease, Harris, aware that only frost or fire can kill cholera, sets several houses aflame. Furious, Tamura places Harris under house arrest and orders him to leave on the next ship. Okichi, now recovered, is disconsolate at the thought of Harris' impending departure. Embittered by months of failure, Harris packs his belongings, but when the epidemic abruptly ends, the people come to thank him for saving their lives, and Tamura tells Harris that, as a debt of gratitude, he has arranged for him to visit the Shogun. To honor Harris, the villagers form a procession to escort him to Edo. Upon reaching the gates of the city, Harris is ushered into the Great Hall of the Shogun, which has been closed to foreigners for centuries. Brandishing the treaty, Harris makes a plea to pull down the barriers existing between Japan and the United States. At a banquet the following day, members of the Council who will vote on the treaty question Harris about the warlike propensity of the West and the practice of slavery. When a Council member observes that it is better for the countries to remain apart, Harris argues for progress. At an archery exhibition the next day, Lord Shido, one of the supporters of the treaty, is assassinated, and Tamura, warning that only violence will come of the treaty, begs Harris to leave immediately. Swayed by Lord Shido's death, the Council passes the treaty. Tamura's clan, opposed to the agreement, orders Tamura to kill Harris, and Tamura enlists Okichi in the plot, reminding her of her unswerving obedience to him. When Harris promises to return to Okichi and alludes to marriage, she is disconsolate because of her promise to Tamura. That night, Okichi, following instructions, ties a red scarf to Harris' bedroom door, signaling Tamura that Harris is sleeping inside. Stealthily entering the house, Tamura enters the room and unsheathes his sword, but when he throws back the bed covers, he finds Okichi and flees. Encountering Harris in the hallway, Tamura slashes the scarf in half, tells Harris to take back his life and then runs out the door. After Okichi informs Harris that Tamura must kill himself for failing his mission, Harris finds Tamura dead in the courtyard, a victim of his own sword. Okichi, aware that she must be punished for breaking her vow to Tamura, forsakes Harris, leaving behind a mirror, the symbol of her soul. As Harris is reverently carried through the streets to sign the treaty, Okichi watches from the crowd, tears in her adoring eyes.