As reverent as he was to the material, Heisserer knew he'd have to make changes. In Chiang's source material, the aliens never touch down on Earth, communicating with Louise (Amy Adams) via a computer screen. But at the core was an emotional story of loss and companionship, memory and fortitude, riding a mind-bender finale that sees Louise discover the alien race's secret: a psychic, simultaneous understanding of past, present, and future.
Building to Louise's awakening, and the implementation of that power to save Earth from a trigger-happy war, meant adding that One Big Line. Where a summer tentpole might send a hero diving for a red "STOP" button or popping two bullets in the villain's back just in the nick of time, Arrival's big climax is phoned in -- literally.
To sway General Chang (Tzi Ma) from launching a ballistic attack on the alien ships, Louise drifts through space-time for answers. We see her flash forward to meet the general at a fancy gala, where he thanks her for reminding him of his wife's dying words. Then, back in the main timeline, she dials him on his personal cellphone while barricaded in a government base airlock and utters his wife's words back to him. The wisdom resonates enough for him to pull back his forces. The alien ships take off. Their work here is done. It's a bit mind-boggling, but the simple explanation is that, with the past, present, and future at her disposal, Louise is able to retrieve her answers from a timeline where she'd already delivered it.
The sequence is even more challenging because we're never given the dialogue MacGuffin. Heisserer says Villeneuve debated whether or not to include subtitles for the line. The director opted to revel in the mystery. The writer wasn't as keen on keeping it a secret, and was happy to divulge. As he told the audience at Fantastic Fest, the line translates to: "In war there are no winners, only widows."
"I worked so hard on the dialogue in Mandarin for Denis," Heisserer wrote on Reddit. "Spent weeks crafting the lines that he finally approved! And then that scoundrel goes and doesn't use subtitles in that scene. I guess there's something to be said there about the nature of language. And I love Denis. But he's also a mischievous fox."