The mother, linguist Louise Banks, is lecturing at a Massachusetts university when twelve extraterrestrial spacecraft appear at twelve different locations across Earth. U.S. Army colonel G.T. Weber asks Louise to join physicist Ian Donnelly and find out why they have come. She is brought to an army camp in Montana near one of the spacecraft.
They make contact with two seven-limbed aliens, whom they call "heptapods"; Ian nicknames them Abbott and Costello. Louise discovers that they have a written language of complicated circular symbols, and she begins to learn a basic vocabulary. As she becomes more proficient, she starts to have visions of herself with her daughter and of their relationship with the absent father.
When Louise finally asks why the aliens have come to Earth, they answer "offer weapon". An alternate translation of "use weapon" is made at another site. This leads China to break off communications with the rest of the world; other nations also stop exchanging information. However, Louise argues that the symbol interpreted as "weapon" might mean "tool".
Rogue soldiers plant a bomb in the spacecraft. Unaware of this, Louise and Ian re-enter the ship. The aliens give them a much larger, more complex message. Just before the bomb explodes, Abbott ejects Ian and Louise from the craft, leaving them unconscious. When Louise and Ian reawaken, the military is preparing to evacuate, and the spacecraft rises and hovers out of reach. Ian discovers that the symbol for time is throughout the message, and that the writing occupies exactly one-twelfth of the space. Louise suggests this means the aliens must want nations to cooperate.
Meanwhile, China's General Shang has issued an ultimatum to the aliens, demanding they leave within 24 hours, and he prepares to attack. Russia, Pakistan, and Sudan follow China's lead. Louise believes that evacuating is a mistake, and that further attempts to communicate are needed. She goes out alone to the alien spacecraft, and it sends down a shuttle to transport her inside. When only Costello appears, she asks about Abbott and is told Abbott is dead or dying. Louise then asks who is the child in her visions; Costello explains that she is seeing the future (her "visions" are not flashbacks, but flashforwards). Costello adds that they have come to help humanity, because in 3,000 years they will need humanity's help in return. It is their language that is the "weapon" or "tool"; those who master it have their perception of time altered, and they can see the future.
Louise returns as the camp is being evacuated. She has a vision of herself at a future United Nations event, being thanked by Shang for making him decide to call off the Chinese attack. He tells her that she called his private telephone number, and he shows it to her. In the present, Louise steals a satellite phone and calls him, but does not know what to say. Her vision continues with Shang explaining that she convinced him by repeating his wife's dying words in Mandarin, which he tells Louise.[nb 1] In the present, Louise recites those words to Shang. The Chinese hold an emergency press conference to announce they are standing down militarily and are releasing their twelfth of the message. Russia does the same and others follow. The twelve spacecraft then disappear from Earth.
During evacuation of the camp, Ian expresses his love for Louise. They consider life choices, and whether they would change them if they could know the future. Louise knows already that Ian will father her daughter Hannah, but that he will later leave her when she reveals she knows Hannah will die prematurely. She also knows that, when Ian will ask her if she wants to "make a baby", she will agree, despite knowing their fates.
