Plot

During World War II, 855 women joined the fight to fix the three-year backlog of undelivered mail. Faced with discrimination and a country devastated by war, they managed to sort more than 17 million pieces of mail ahead of time.

During the height of the conflict in World War II circa 1943, difficulties in the war led to a change in priorities, and much of the mail previously reaching soldiers on the front lines has been halted. As warehouses begin to overflow with unfulfilled deliveries, soldiers' hopes are dashed as they lose contact with their families back home. To fix this problem, the 6888th Battalion the Women's Army Corps' all-Black battalion including over 800 female soldiers is sent to tackle the impossible challenge of sorting years' worth of backlogged mail. Even as military higher-ups hope to see the plan fail, the Six Triple Eight are determined to deliver hope to soldiers and families across the world.

San Pietro, Italy, December 1943. An American unit comes under heavy fire from a German fortification. A plane crash lands in the middle of the battle. A soldier pulls the dead pilot out of the wreckage and finds a blood-soaked letter on his person. After the fighting ends, the soldier heads back to his base, and drops the pilot's letter in the mail bag, to be delivered back home. From there, all mail bags are delivered to a sorting center. A West Virginian mother waits every day for news about her two sons who are serving overseas.

Meanwhile childhood friends Lena (Ebony Obsidian) and Abram (Gregg Sulkin) reckon with the difficulty of their budding romance (Lena is African American and Abram is Hebrew) in 1940s Philadelphia, as he prepares to deploy. Lena lives with her mother Emma (Donna Biscoe) and Aunt Susie (Baadja-Lyne Odums). Abram dies shortly after enlisting and a heart-broken Lena vows to enlist after her high school graduation. The army is also enlisting women, as they are short of people to do back-end jobs. Lena's mother and aunt try to counsel her that African American women will never be given any other job than cooking or cleaning. They want Lena to enlist in university.

On the train to basic training in Fort Oglethorpe Georgia (in 1944), Lena meets other enlistees who will become her friends including Johnnie Mae Burton (Shanice Shantay), Bernice Baker (Kylie Jefferson), Dolores Washington (Sarah Jeffery) and Elaine White (Pepi Sonuga). There, the battalion is trained by Captain Charity Adams (Kerry Washington) and Lieutenant Noel Campbell (Milauna Jackson). Charity maintains a high standard for the unit and says that African American units in the army have the burden to be better than their white counterparts. Despite being well-trained, the battalion is never given any orders. Lena also meets Private Hugh Bell, who is interested in her.

One day, the West Virginia mother travels to the White House to meet Eleanor Roosevelt (Susan Sarandon) and tells her that, across the country, no one is receiving mail from the war. President Roosevelt (Sam Waterston) meets with two high-ranking officers, Mrs. Roosevelt, and Mary MacLeod Bethune (Oprah Winfrey) to discuss the situation. Mary MacLeod Bethune is on the National Council of African American Women and has a seat at the President's African American cabinet and is close friends with Mrs. Roosevelt.

General Halt admits that mail is not being delivered but explains that, despite assigning several units to the job, including a women's unit, the task is a logistical nightmare, and they have to prioritize actual fighting. However, Halt also admits that only the White Women Army Corps have been deployed to sort out the mail. He argues that the job is complicated and requires intelligence. Bethune maintains, however, that the African American Women's Army Corps is up to the task, so Captain Adams' unit is finally given orders.

In Europe, the battalion faces a racist commanding officer, General Halt (Dean Norris), who has the unit cross the ocean in a private vessel without a military escort in February 1945. Then they are to march through town immediately upon landing in Glasglow, Scotland, and assigned to a disused, rat-infested, unheated grammar school. The 6888 Battalion has 855 women.

The newly promoted Major Adams and her second-in-command Captain Campbell are given six months to turn the school into a post office and barracks and clear a two-year backlog of undelivered mail. The deadline is intended to ensure their failure. Charity takes command of her Battalion and establishes a system to process the mail at their base at the school, once it is delivered to them on trucks from the hangar.

Once the battalion has toiled to put together a livable barracks, pristine mess hall and well-organized mail sorting hall, one day Lena loses her composure upon seeing nonchalant handling of dog tags. Later, she breaks down to a few close mates, revealing her connection to Abram was her reason for enlisting. As she had not received any correspondence from him, she hopes to find some now. Major Adams, now knowing this first-hand response toward the mail blockage, feels inspired.

Along with a notification that their first two months of sorted mail was not deliverable, the battalion is asked to host African American soldiers to boost morale. 57,000 pieces of mail were shipped out, and none of them delivered as the soldiers were moving around a lot at the front. Charity suspects that the Battalion is being given bad locator cards for the soldiers.

Lena sees Hugh (Jay Reeve) there, who she had met previously. They start to dance, but she soon leaves upon recognizing a song she associates with Abram. So, they decide to be friends.

When it is discovered that an occasional letter is returned to sender, the battalion members point out various difficulties and challenges they face. These include multiple cases of identical names, multiple locator cards for soldiers as their units move, rats are damaging addresses and their contents and mold, among other things.

After FDR's death, a white chaplain Clemens (Nick Harris) arrives and delivers a sermon which undermines Major Adams' qualifications and morals. She promptly finds the extremely negative report he has written about her, has his things packed and sends him off. Lena figures out that the soldiers are putting the symbol of their division masthead on their letters, and that is a quick way to identify where they are.

On the day Lena gets Abram's letter (turns out the dead pilot at the beginning of the film, who had the letter on him was Abram), two women of their unit die from a UXB bomb in route back to the school. After their burial, Lena is able to visit his grave, reading his letter asking her to live a long life and finally gives him a proper goodbye.

General Halt visits the 6888, criticizing them harshly. As they work around the clock, the night shift is sleeping when he arrives. He is not allowed in the showers lest someone is showering. After Halt inspects the mail sorting area, he declares Major Adams incompetent and says he will replace her with a white male. Adams responds, "over my dead body", on which she elaborates eloquently in the General's face, makes clear that she will not step down. The soldiers who heard the exchange remain standing at attention until she comes down to get them to start work again, whereupon they give the Major a rousing applause.

As General Halt is trying to push Major Adams' court martial through, he hears cheers from his unit as their mail finally arrives. The 6888 successfully straightened out the backlog of two years or 17 million pieces of mail in just 90 days, despite the challenges. They were later sent to Rouen, France, to clear another backlog. Lena later marries Hugh, living a long life.

Not especially appreciated when returning to US soil at the time, Michelle Obama oversees a ceremony honoring the 6888. Also, US Army Base Fort Lee was renamed partially in Adams' honor.