The film serves as a prequel, detailing the very beginning of the alien invasion and the immediate collapse of civilization, focusing specifically on the chaos that unfolds in New York City.

The protagonist is Samira (Lupita Nyong'o), a sharp, cynical woman who has come to New York for a day trip. She is completely unaware of the global catastrophe beginning to unfold. She finds herself in the middle of the crowded, noisy city when the first wave of extra-terrestrial creatures, later known as the "Death Angels," arrives.

The invasion begins abruptly. The creatures, blind but possessing hypersensitive hearing, descend upon the noisy metropolis. The initial response is mass panic screaming, car horns, and explosions which immediately draws the creatures, leading to unimaginable devastation.

Samira is trapped in the city and forced to quickly adapt to the terrifying new reality. She realizes that silence is the only key to survival. She manages to link up with Eric, a traumatized stranger who has a deep, almost encyclopedic knowledge of the city's hidden pathways and underground infrastructure.

Their primary mission becomes a journey across the rapidly emptying, silent city to reach a designated evacuation point a quiet zone established briefly by the military on the outskirts of Manhattan. The journey is fraught with continuous near-misses, forcing them to communicate solely through gestures, sign language, and extremely slow movements.

Along the way, they encounter several characters who fail to grasp the necessity of absolute silence, leading to fatal consequences that reinforce the rules of this new world. Samira, having been deafened in one ear in a previous accident, finds that her partial hearing loss paradoxically offers her a slight advantage, allowing her to better filter the surrounding absolute silence.

A secondary threat emerges from desperate human survivors who become dangerously territorial over resources and quiet hiding spots. Samira and Eric must contend not only with the alien monsters but also with the savagery of their fellow humans.

The climax involves reaching the evacuation point, which is revealed to be compromised and under heavy attack. They find themselves in a final, intense sequence where they must use the surrounding urban environment such as diverting a stream of running water or using structural collapse noise to draw the creatures away from their escape route without making any personal sound.

The film concludes with Samira and Eric successfully escaping Manhattan, but their sanctuary is brief. They realize that "Day One" is over, but the war for quiet survival has just begun, leaving them and the audience to face an unknown future in a world fundamentally changed by silence.