Jason Delgado, an unemployed engineer, finds himself facing an extraordinary situation while enjoying a peaceful day fishing. The calm is abruptly interrupted when a strange-looking storm approaches. As rain begins to pour, he rushes back to his suburban Seattle home, where he lives with his wife Ann, along with their two children, Graham and Ruth, and their beloved family dog, Cassidy. However, as they attempt to leave their home, they quickly discover that all exits - doors and windows - are mysteriously sealed shut. To make matters worse, their Internet and phone lines are down, leaving their neighbors in a similarly dire predicament, with some people stranded outside.
In a state of panic, Jason attempts to break through the barriers surrounding them, but it becomes clear that the water forming outside has created an impenetrable barrier. Fortunately, they still have electricity and running water but after two grueling days without any sign of help, Jason and Ann realize they need to ration their supplies—food, water, and electricity—from the power grid. They initiate various activities to keep Graham and Ruth engaged, including lessons on essential survival skills.
As the days turn into weeks, Ann dedicates her time to cultivating an indoor vegetable garden while Jason focuses on devising a water reclamation system. Ruth, curious and imaginative, spots a creature lurking outside during the rainy weather, believing it to be friendly. However, when she shares her thoughts with her family, they dismiss her observations as mere childish fantasies.
Months pass without any reprieve, and one day, a frantic Susan, their neighbor, arrives at the house, yelling that there’s no one left outside. Unfortunately, she is soon attacked by one of the menacing creatures, which reinforces Jason and Ann's decision to conceal their presence during rainfall, as that is when the creatures are most active. When the chimney cap unexpectedly falls in, they attempt to hoist Ruth up through the chimney. Although she cannot go outside, she verifies that the opening does lead to the outside world. Tragically, Cassidy falls ill when his medication runs out, leading Jason to make the painful decision to end Cassidy's suffering, and he stores the remains in the freezer for future use.
Innovatively, Jason, with the help of Graham, builds traps that they send out through the chimney opening to capture small animals roaming in the suburb. They utilize Cassidy’s remains and some of the vegetables from Ann’s garden as bait. Eventually, the power grid collapses, adding to their struggles.
Five years after the initial storm, they have managed to expand Ann's garden through the living room floor and develop a functional water system throughout their home. Despite these efforts, the emotional weight of survival takes a toll on Graham and Ruth, as they feel utterly defeated. One day, a creature with tentacles and a body dripping with seawater invades their home. Although it doesn’t find them, it contaminates their food and water supplies. In a fateful moment, Ruth encounters the creature’s liquid, leading her to taste it and realize it is saltwater.
Complications arise when Ruth contracts an infection after being bitten by a ferret. Driven by a desperate need for fresh air and freedom, Jason discovers cracks in the home’s flooring that signal an opportunity for escape as the house sinks and the water level shifts. Ann bravely ventures through the sewage pipes to retrieve antibiotics from their dentist neighbor, Max Richardson. Unfortunately, she is confronted by a creature during her escape, forcing Jason to seal the opening behind her for their safety. Knowing they are at risk of being attacked, the family begins to prepare weapons and traps.
As rain pours once more, water begins to flood their house from the pipes, driving the family upwards into the attic. A creature manages to breach their defenses, leading to a harrowing confrontation. Jason hesitates to kill it at Ruth's insistence, showcasing mercy in the face of danger. This act of kindness surprisingly prompts the creature to open the front door, allowing the family to leave, while it pleads with the other creatures to spare them and the planet from further harm. Just as they escape, the house succumbs to a sinkhole, disappearing into the earth.
With the rain finally halting, the Delgados emerge to find Seattle almost entirely submerged in water. Determined to survive, the family sets out on a salvaged boat, embarking on a journey across a now-flooded world in search of other survivors, using a radio to reach out for help and connection.
