Brothers Justin and Aaron find themselves unexpectedly drawn back to their youth when they receive a video cassette from their former group, Camp Arcadia. While the two brothers have differing memories of their time at the camp—Justin believes it was a UFO death cult, while Aaron recalls it as a quaint and friendly commune—the video suggests that the members might still be alive. Despite Justin's concerns about the ominous talk of "ascension," which he worries could signal a future mass suicide, Aaron is eager to revisit Camp Arcadia for just a single day, feeling frustrated by their current lives filled with job rejections and lonely nights.
Upon their arrival, they are warmly welcomed back to Camp Arcadia, where they realize that none of the members have aged a day since their departure a decade ago. Aaron catches the eye of Anna, while Justin keeps his distance, focusing solely on his brother. One of the camp members, Hal, shares his excitement with Justin about a physics equation he has been developing, though he admits he can't fully explain it—he likens it to conveying a concept beyond the perception of color. This leaves Justin skeptical yet curious as they engage in various camp activities. As the day continues, Aaron becomes more enchanted by the commune's charm, persuading Justin to extend their stay for another day.
As their exploration deepens, peculiar and unsettling experiences start to unfold. During a tug-of-war challenge with a rope that inexplicably ascends into the night sky, Justin can't logically explain his defeat, heightening the atmosphere of mystery. The brothers begin to notice strange occurrences; while wandering through the woods, Justin feels the presence of an unseen entity, which leads him to a photograph of a buoy. In conversation with Hal, he learns that the man does not have all the answers—his own equation is merely a personal interpretation of what is happening. He suggests that Justin explore the lake beneath the buoy featured in the photo. As two moons rise in the sky, Hal warns Justin to come to a resolution before a third moon makes an appearance.
While fishing, Justin dives into the water after spotting the buoy from the photograph. He resurfaces with a toolbox, claiming that something attempted to drag him down, and upon opening it, they discover a tape that seems to document Justin and Aaron misrepresenting Camp Arcadia during a past interview. Tensions flare when Hal plays the tape, as Justin accuses him of being a cult leader, which leads to an explosive confrontation between the two. Aaron, feeling conflicted and unwilling to leave his newfound community behind, chooses to stay, regardless of Justin's insistence on leaving.
As Justin ventures away to seek help for their car, he discovers individuals trapped in a time loop—a surreal predicament in which they repeatedly face their violent ends for the amusement of the entity. They warn him that he, too, will be ensnared when the third moon rises. Eventually, he finds Aaron, who reveals his desire to remain, believing that experiencing death at the hands of the entity would be a more fulfilling life than returning to menial jobs filled with isolation. Realizing his error in pressuring Aaron into this lifestyle, Justin comes to understand that they must escape together. Just in the nick of time, they narrowly evade catastrophe as the entity destroys Camp Arcadia. In a tense moment, Justin urges Aaron to fill the gas tank, but Aaron stands baffled, pointing out that their fuel gauge has always read empty.
