Four astronauts—Captain Theo Cooper, Dr. Emily McTier, Bug Kieslowski, and Cole Dvorak—embark on an ambitious journey to undergo a 400-day-long simulation on Earth designed to analyze the long-term effects of space travel. As they spend their time in isolation underground, the crew faces increasingly severe psychological challenges, including paranoia and hallucinations. As tension grows, each member begins to retreat into their own space, both physically and mentally.
One fateful day, a disheveled man unexpectedly appears in their simulated spacecraft, breaking through the boundaries of their isolation. After his sudden departure, the astronauts make the bold decision to exit their underground haven and explore the outside world, only to discover a landscape that has drastically changed. They find the Earth to be enveloped in darkness, with howling winds and an eerie sense of desolation as dust clouds shroud everything in an oppressive gloom.
Venturing forth, they stumble upon a town called "Tranquillity," which surprisingly boasts inhabitants and electricity. A local man named Zell invites the weary travelers into his diner, where he explains that the moon was hit by a colossal object that unleashed a massive dust cloud, now blocking out sunlight and drastically altering their environment. Despite this alarming revelation, Dvorak remains skeptical, convinced that they are still caught in the simulation, and chooses to venture off to a nearby bar, a decision soon followed by his comrades.
After indulging in a few drinks, Dvorak leaves with a young woman, opting to disregard the safety of his crew. The remaining trio returns to the diner, where Zell extends an invitation for them to rest the night. Bug, taking the first watch, soon finds himself lost in hallucinations, believing he is following his son, Sam, yet unwittingly challenges Zell in the process. When McTier and Cooper awake, they are alarmed to discover that their friends have vanished. When they question Zell about the missing crew members, he curiously claims that they arrived alone, raising suspicion and alarm in McTier and Cooper.
Fearful for their well-being, the duo sets out to find their comrades but is unable to locate them. After a frantic search, they return to their ship, only to realize that they are being trailed by Zell and two other sinister men. A tense confrontation ensues, culminating in Cooper and McTier successfully neutralizing the threats. In a twist of fate, the moment Zell takes his last breath coincides with the conclusion of the 400-day simulation, which the astronauts receive notification has ended successfully.
As the entrance hatch opens, they are greeted by a blinding light, prompting Cooper and McTier to grasp each other's hands tightly, their hearts racing with anticipation of what lies ahead. The film closes on this moment of uncertainty, leaving audiences on the edge of their seats, wondering about the fate that awaits them beyond the simulation's end.
