Emily Liana Liberato and Randall Noah Le Gros arrive at Randall's parents' beach house in a remote vacation town, hoping for a quiet escape. The town looks eerily deserted as they step inside, and their mood lightens with the idea of a private getaway. Soon they discover Mitch Jake Weber and Jane Turner Maryann Nagel—old friends of Randall's father—are already staying there. Jane is terminally ill, and Mitch wants to give her one last happy vacation. With plenty of space at the seaside house, the two couples decide to share the residence for a while, aiming to make the best of a peaceful retreat.
After dinner, Randall brings out some edible marijuana, and the group agrees to share. As they drift into a high, Mitch notices the strange texture of the water, and everyone spots glowing microbes on the beach and in the trees. Jane steps out to observe the phenomenon up close, but the fog begins to form and she becomes disoriented while coughing fits intensify. Emily also grows lightheaded and slips into unconsciousness. When she awakens, she finds Jane in the kitchen, catatonic and covered in skin lesions, while Mitch has vanished.
The next day, Emily and Randall head to the beach and are stunned to find it completely empty. Randall soon becomes sick and hurries back to the house. In the bathroom, he notices the sink water moving in peculiar patterns; at the same time, Jane moans softly somewhere nearby. On the shore, Mitch reappears and muses that something in the area feels off before he goes for a swim—and then inexplicably drowns himself. A panicked Emily steps on a washed-up Portuguese man o' war, gets stung, and spots a worm-like creature wriggling in an open wound. She fights her way back to the house, where she finds Jane sickeningly transformed and stalking with milky white eyes. She manages to lock Jane inside and, with Randall now gravely ill, the two flee into the thickening fog.
By nightfall, the fog has blanketed the landscape, and Randall deteriorates further. Emily discovers a police car with a radio and manages to reach out to someone who warns them to barricade themselves and avoid breathing the fog. They move from house to house, witnessing the water’s texture change in unsettling ways, and TVs and radios flood with warnings: the microbes—preserved in underwater rocks—have been released, apparently due to global warming. Emily proposes a plan to survive until morning, but Randall’s fear pushes him toward a desperate course. In the basement, she finds oxygen tanks for scuba diving and deduces they can use the tanks to breathe air in a safe space. She also encounters a horribly mutated person feeding on a corpse, a grim sign of the infection spreading.
Upstairs, Randall convulses and vomits a squid-like creature as his eyes turn white. Emily returns with the scuba gear and, when he lunges at her, she defends herself by striking him with a scuba tank. With the keys to a car in hand, she drives away, but the fog roils in and she becomes disoriented, crashing into a tree. She collapses into a tide pool, crawling out into the fog-drenched air and slowly breathing in the eerie atmosphere. She experiences a series of dreams and hallucinations about the microbes as dawn breaks and she lies on the beach, trying to steady herself as the infection completes its course. A final, overwhelming wave surges over her body and drags her out to sea, leaving her fate uncertain as the town sleeps under the fog.
