In a retro-futuristic version of the 1950s, a television host introduces a live production of Asteroid City, a play in the film's setting by famed playwright Conrad Earp. In the play, a youth astronomy convention is held in the fictional desert town of Asteroid City. The play's events are depicted in widescreen and stylized color, while the television special is seen in black-and-white Academy ratio.In the play, war photojournalist Augie Steenbeck arrives early to the Junior Stargazer convention with Woodrow, his intellectual teenage son, and his three younger daughters. When their car breaks down, Augie phones his father-in-law, Stanley, asking his help. Stanley, who dislikes his son-in-law, persuades him to tell the children about their mother's recent death, which Augie had concealed. Augie and Woodrow meet Midge Campbell, a famous but world-weary actress, and her daughter Dinah, who, like Woodrow, will be honored at the convention. Augie and Midge, and Woodrow and Dinah, gradually fall in love throughout the play. The other convention participants arrive: five-star General Grif Gibson, astronomer Dr. Hickenlooper, three additional teenaged honorees (Ricky, Clifford, and Shelly) and their parents (J.J., Roger, and Sandy), a busload of elementary-school children chaperoned by young teacher June Douglas, and a cowboy band led by singer Montana. A local motel provides everyone's accommodations.Gibson welcomes the attendees at the Asteroid City crater where the teenagers are to receive awards for various inventions. A UFO suddenly appears above the crater; an alien emerges and steals a fragment of the meteorite that created the crater. Augie photographs the alien. General Gibson with instructions from the president, places the town under military quarantine, and everyone is subjected to medical and psychiatric examinations. Meanwhile, a romance blossoms between Montana and June, who assure the students that the alien is likely peaceful. The Stargazer honorees use Dr. Hickenlooper's equipment to attempt to contact the alien. Using a guarded pay phone, Ricky calls his school newspaper to relay the quarantine details and cover-up to the outside world.The Asteroid City events become national news. A furious General Gibson is about to end the quarantine when the UFO reappears and the alien returns the meteorite fragment. When Gibson reinstates the quarantine, the children, scientists, and parents revolt, using the honorees' inventions to overpower the military.The play's creation is interspersed between the play. Some time after Conrad Earp started writing, he meets with actor Jones Hall, who performs an audition in Earp's home and is immediately cast. During the same interaction, Earp and Hall kiss, establishing their relationship as lovers. Earp writes the play with help from a local acting school and recruits most cast members from it, including Mercedes Ford, a temperamental yet talented actress who plays Midge.During the recorded performance of the play, Hall, who plays Augie, confronts the play's director Schubert Green, saying he "still doesn't understand the play", and asks Green if he is "doing him right". Green tells Hall to keep playing Augie the same way despite being uncertain, and that he is doing him right. After that interaction, while taking a smoke break on a balcony, Hall runs into the actress who was cast to play Augie's wife before her only scene was cut. She recites the deleted scene's text to him.Six months into the play's run, Conrad Earp dies in an automobile accident. In the play's epilogue, Augie and his family are the last to leave Asteroid City after General Gibson lifts the quarantine. Augie's daughters bury their mother's ashes in the desert, Woodrow wins the fellowship funding, and Midge leaves Augie her mailing address. Augie and his family quietly drive away.