[2] The film tells the story of how a small suburban town near the San Francisco Bay Area slowly falls apart after a nuclear war destroys outside civilization.

The cast includes Jane Alexander, William Devane, Leon Ames, Ross Harris, Lukas Haas, Roxana Zal and, in small roles shortly before their rise to stardom, Kevin Costner and Rebecca De Mornay.

It was one of the films, along with The Day After and Threads that portrayed life after a nuclear war, mostly in response to an increase in hostility between the United States and the Soviet Union.

Scottie watches Sesame Street on TV when the show is suddenly replaced by white noise; after a few seconds, a San Francisco news anchor appears onscreen, saying they have lost their New York signal and there are radar indications of explosions of "nuclear devices up and down the East Coast."

An announcer's voice states that the White House is interrupting the program, but the blinding flash of a nuclear detonation is seen through the window and the broadcast goes dead.

The family huddles on the floor in panic as the town's air-raid sirens go off; minutes later, several of their neighbors are seen running around on the street outside, dazed in fear and confusion.

As the dead accumulate faster than the manpower to bury them, wooden caskets are used as fuel for funeral pyres later, and Carol sews together a burial shroud from bedsheets for her daughter, Mary Liz, who also dies from radiation exposure.

While the young die, older residents fall to rapid dementia, and order in the town starts to break down as police and firefighter ranks dwindle.

With no one to renovate homes and no purpose to do so, properties, roads, and the physical appearances of survivors steadily deteriorate as radiation poisoning and social disintegration take their toll.

To her sorrow, she finds a later (and previously unheard) message on the machine from Tom: he decided to stay at work late in San Francisco on the day of the attack, and she gives up hope that he will return home.

Kyle Raymond Fitzpatrick of Los Angeles Review of Books traced Amen's story to a number of other 1980s periodical publications, stating, "The appearance of Amen’s “The Last Testament” in [Ms. Magazine] was actually a reprint: the story originally ran in the September 1980 issue of the St. Anthony Messenger, an Ohio-based religious magazine published by the Franciscan Friars, with a circulation of roughly 350,000 in the 1980s.

Screen Rant has listed the film as one of the best science fiction movies directed by a woman, noting that it "went on to inspire other speculative war disaster media such as the TV show Jericho.

"[16] Tor.com, in a piece on dystopian science fiction, called Testament "a realistic vision of the world we might live in after the bombs drop" and "quiet, brilliant little movie.

"[18] The Encyclopedia of Science Fiction calls it "potent and sentimental" and notes that it "seems not interested in causes, only in effects – in marked contrast to The Day After, made the same year.