Emily and Randall arrive at Randall's parents' beach house, tucked away in a remote vacation town that already feels isolating even before they step inside. The town appears deserted, and the couple settles into the quiet with a mix of relief and curiosity. Upstairs, they share a private moment, then drift into a minor disagreement, only to learn that another couple, Mitch and Jane Turner, has rented the place as well. Jane is terminally ill, and Mitch hopes to gift her one last, carefree escape. With plenty of room in the house, the two couples decide to spend their time together, easing into a shared rhythm away from the outside world.
That evening, Randall brings out some edible marijuana, and the group agrees to share. The mood lightens, but the atmosphere soon grows strange. While everyone is high, Mitch notices an unusual texture to the water and discovers glowing microbes coating the beach and the surrounding trees. Jane insists on getting a closer look, but the moment she steps into the fog, she begins coughing and becomes disoriented as a dense mist sweeps through the area. Emily feels the same disorientation and passes out. When she comes to, she finds Jane in the kitchen, catatonic and covered with skin lesions, and Mitch has disappeared.
As the sun rises, Emily and Randall venture to the beach only to find it empty and unnerving. Back at the house, Randall falls ill and rushes to the bathroom, where the water in the sink forms bizarre patterns. Hearing Jane moan nearby, he hurries to her side. Outside, Mitch inexplicably reappears and mutters that something in the area feels off before claiming he will swim. He dives into the waves and drowns. Emily, in a panic, steps on a washed-up Portuguese man o’ war and is stung, discovering a worm-like creature wriggling inside a wound. She drags herself back to the house and manages to remove the worm. Inside, Jane advances from illness toward something far more unsettling, her eyes turning milky white and fluids leaking from her mouth. They lock Jane in the house and flee into the thickening fog.
Night falls, and the fog thickens into an almost tangible presence. Randall grows sicker, and Emily discovers a way to contact help through a police car with a radio. The voice on the other end warns them that they have been exposed and instructs them to barricade themselves and avoid breathing the fog. They barricade a house and realize the water outside has taken on an entirely new texture. A broadcast on every channel confirms their fears: the tiny microbes, once sealed away in underwater rocks, have been released by global warming. Emily suggests waiting until morning, but Randall’s fear over the infection drives him toward reckless desperation. She retreats to the basement, where she finds scuba oxygen tanks and the means to breathe safer air. She also stumbles upon a horrifying mutated person feeding on a corpse.
Upstairs, Randall vomits a squid-like creature, and his eyes turn white as the infection takes hold. Emily returns with the scuba gear, but she must defend herself as Randall lunges at her. She strikes him with a scuba tank to stop him and seizes the car keys, hoping to escape. The fog confuses her as she drives away, and a crash into a tree leaves her disoriented. She inches toward a tide pool, breathes the damp, foggy air, and begins to hallucinate about the microscopic invaders in the water. She collapses on the shore as dawn breaks, insisting to herself that she can endure what comes next. The sea answers her resolve with a final, relentless embrace: a wave pulls her body under and out to sea, leaving the fate of Emily—and the town—unclear but sealed in the relentless advance of the infection.
