Linguist Louise Banks's daughter Hannah dies at the age of twelve from an incurable illness.
Twelve extraterrestrial spacecraft hover over various locations around the Earth. In the ensuing widespread panic, affected nations send military and scientific experts to monitor and study them. In the United States, US Army Colonel Weber recruits Banks and physicist Ian Donnelly to study the craft above Montana. On board, Banks and Donnelly make contact with two cephalopod-like, seven-limbed aliens, whom they call "heptapods"; Donnelly nicknames them Abbott and Costello. Banks and Donnelly research the complex written language of the aliens, consisting of palindromic phrases written with circular symbols, and share the results with other nations. As Banks studies the language, she starts to have flashback-like visions of her daughter.
When Banks is able to establish sufficient shared vocabulary to ask why the aliens have come, they answer with a statement that could be translated as "offer weapon". China interprets this as "use weapon", prompting them to break off communications, and other nations follow. Banks argues that the symbol interpreted as "weapon" can be more abstractly related to the concepts of "means" or "tool"; China's translation likely results from interacting with the aliens using mahjong, a highly competitive game.
Rogue soldiers plant a bomb in the Montana craft. Unaware, Banks and Donnelly reenter the alien vessel, and the aliens give them a more complex message. Just before the bomb explodes, one of the aliens ejects Donnelly and Banks from the vessel, knocking them unconscious. When they wake, the alien craft has moved beyond reach and the US military is preparing to evacuate in case of retaliation.
China's General Shang issues an ultimatum to his local alien craft, demanding that it leave China within 24 hours. Russia, Pakistan, and Sudan follow suit; communications between the international research teams are terminated as worldwide panic sets in.
Donnelly discovers that the symbol for time is present throughout the message and that the writing occupies exactly one-twelfth of the 3D space into which it is projected. Banks suggests that the full message is split among the twelve craft and that the aliens want all the nations to share what they learn.
Banks goes alone to the Montana craft, which sends down a transport pod. Abbott has been mortally injured as a result of the explosion. Costello explains that they have come to help humanity, because in 3,000 years' time they will need humanity's help in return. Banks realizes the "weapon" is their language. Learning the language alters humans' linear perception of time, allowing them to experience memories of future events. Banks's visions of her daughter are revealed to be premonitions; her daughter will not be born until sometime in the future.
Banks returns to the camp as it is being evacuated and tells Donnelly that the aliens' language is the "tool" that was meant by the word "weapon". She has a premonition of a United Nations event celebrating newfound unity, following the alien arrival, in which General Shang of China thanks her for persuading him to stop the attack when she called his private number and recited his wife's dying words.[a] He then shows her his private number and whispers his wife's words into her ear.
In the present, Banks takes CIA agent Halpern's satellite phone from a table and calls Shang's private number to recite the words. The Chinese announce that they are standing down and releasing their twelfth of the message. The other countries follow suit, and the twelve spacecraft depart.
Based on what she has learned, she writes and publishes a book called The Universal Language, a guide to the heptapod language, which will eventually teach humanity to perceive time the same way as the heptapods.
During the evacuation, Donnelly expresses his love for Banks. They talk about life choices and whether he would change them if he could see his life from beginning to end. Banks knows that she will agree to have a child with him despite knowing their fate: that Hannah will die from an incurable disease and that Donnelly will leave them both as a result of her revealing that she knew this.
