In what appears to be a flashback scene, linguist Louise Banks is caring for her daughter Hannah, who dies during adolescence from cancer. In the present, while Louise is lecturing at a university, 12 extraterrestrial spacecraft appear across the planet. U.S. Army Colonel Weber asks her to join a team, with physicist Ian Donnelly, to decipher their language and discover why they have come to Earth. The team is brought to a U.S. military camp in Montana near one of the craft, and makes contact with two seven-limbed aliens on board. They call the extraterrestrials "heptapods" and Ian nicknames them Abbott and Costello. Louise discovers that the aliens have a written language of complicated circular symbols, and she begins to learn the symbols that correspond to a basic vocabulary. As she becomes more proficient, she starts to see and dream images of herself with her daughter.
When Louise is able to ask what the aliens want, they answer: "offer weapon". A similar translation "use weapon" is received by one of the other sites. Fear of a potential threat from the aliens leads other nations to close down communications, and some prepare their militaries for attack. However, Louise thinks that the symbol for "weapon" might have an alternative translation, such as "tool".
Rogue soldiers plant explosives in the spacecraft. Unaware, Louise and Ian enter to communicate with Abbott and Costello again. The aliens give them a much larger and more complex message. Abbott ejects Ian and Louise from the craft as the explosion occurs, knocking them unconscious. Louise and Ian awaken in the camp as the military is making plans to evacuate, and the spacecraft moves higher into the atmosphere.
Ian works out that the symbols relate to the concept of time, and that it was one-twelfth of the whole "gift"; they conclude that the aliens must want nations to cooperate.
Meanwhile, China notifies the world that it is ready to attack. Louise rushes back to the spacecraft, which sends down a shuttle to take her inside. She meets Costello, who communicates that Abbott is dying. Louise asks about her visions of a daughter, and Costello explains that she is seeing the future, revealing to her that her "visions" are not flashbacks but flashforwards. Costello also communicates that they have come to help humanity by sharing their language, which is the "weapon" or "tool", because it changes the perception of time. The aliens already know that in 3000 years they will need humanity's help in return.
Louise returns as the camp is being evacuated. She has a new vision of herself at a future United Nations reception, being thanked by General Shang for convincing him to suspend the Chinese attack. He explains that she had called his private telephone. He shows her its number, which he says he knows he must do without understanding why. In the present, Louise steals a satellite phone and calls Shang, but realizes she does not know what to say. Her vision continues with Shang explaining that she had convinced him by repeating his wife's last words in Mandarin, which he tells Louise. After the Chinese attack is called off, the other nations resume contact, and the spacecraft leave Earth.
When packing to leave the camp, Ian admits his love for Louise. They discuss life choices, and whether they would change them if they knew the future. Louise foresees that Ian will father her daughter Hannah, whose name is an intentional palindrome, and leave her after discovering that she knew her daughter would die. Despite seeing that Hannah is destined to die as a young adult, and that Ian will leave her when she eventually reveals their daughter's future, when Ian asks her if she wants to have a baby, she agrees.