When mysterious spacecraft touch down across the globe, an elite team - led by expert codebreaker Louise Banks - is brought together to investigate. As mankind teeters on the verge of global war, Banks and the team race against time for answers – and to find them, she will take a chance that could threaten her life, and quite possibly humanity.
Twelve extraterrestrial spacecraft appear across the Earth. Louise is asked by US Army Colonel Weber to join a team to find out why they have come. Accompanied by theoretical physicist Ian Donnelly, Louise makes contact with two seven-limbed cephalopod-like aliens, which they call "heptapods", and Ian nicknames them Abbott and Costello. Louise discovers that the aliens use a written language of complicated circular symbols. They begin to learn the symbols that correspond to a basic vocabulary. As Louise becomes more proficient in the language, she starts to see images of herself with her daughter.
When Louise is able to ask what the aliens want, they answer: "offer weapon". Similar translations ("use weapon") are deduced at other sites, leading other nations to close down communications, and some to scramble their military believing the message indicates a threat. However, Louise thinks that "weapon" might have an alternative translation such as "tool."
Rogue soldiers plant explosives in the spacecraft. Unaware, Louise and Ian go back inside, where the aliens display an image of hundreds of smaller symbols. Costello leaves just before the device explodes, but Abbott stays and pushes Louise and Ian out of the chamber. They both wake up in the base camp with a concussion, as the spacecraft moves higher into the sky. Ian works out that the pattern of symbols relates to the concept of time, and that it was one twelfth of the whole "gift"; hence nations must co-operate to get all of the information.
Meanwhile, the Chinese prepare to attack their spacecraft. Louise rushes back to the spacecraft in Montana, which sends down a shuttle to take her inside. She meets Costello, who communicates that Abbott is dying. Louise asks about her visions, and Costello explains that she is seeing the future: this reveals that her 'visions' are not flashbacks but flash-forwards. Costello communicates that they have come to "help humanity" by sharing their language, which changes the perception of time, and is the "weapon" or "tool" they offer. The aliens also foresee that in 3000 years time they will need humanity's help.
Louise returns as the camp is being evacuated, and has a new vision of being at a United Nations party to commemorate the alien visit. She sees herself being thanked by Chinese General Shang for convincing him to suspend his attack. Shang explains that she had called his private mobile number, which he then shows her. He says he feels it is important to show her his number, but he doesn't understand why. Back in the present, Louise steals a satellite phone and calls Shang, but realizes that, although she speaks Mandarin, she doesn't know what to say. Her future vision continues with Shang explaining that she had convinced him by repeating his wife's last words, which he tells to Louise. After the Chinese attack is called off, the other nations resume contact with each other, and the twelve spacecraft leave the Earth.
When packing to leave, Ian admits his love for Louise. The film closes as they discuss life choices and whether they should change if you could see the future. Louise sees a vision of Ian as the father of her daughter. Her vision continues with Ian asking her, further into the future, if she wants to make a baby. Louise sees herself replying "Yes," wanting to share a short time with her future child rather than prevent her ever from existing.