Eight friends gather for a dinner party in California while a comet passes overhead. During the evening, tension arises when Kevin invites Emily to join him on a business trip to Vietnam, but she remains noncommittal. Among the guests is Kevin's ex-girlfriend, Laurie, who still harbors feelings for him. As the night progresses, a sudden power outage plunges the home into darkness. The hosts find glow sticks to light their way, opting for blue ones, and venture outside where they notice one house up the street still has power.
Returning inside, the group discovers a broken glass, an unsettling omen they initially overlook. To seek help, Hugh and Amir venture to the neighboring house, soon returning with a box. Inside, they uncover a ping-pong paddle and photographs of themselves, including one taken that very night, all marked with mysterious numbers. Hugh reveals that he has seen a dining setup for eight people at the other house, leading to a startling realization: the other house houses alternate versions of themselves.
Intrigued, Emily tries to deduce patterns from the numbers scribbled on a notepad but finds no substantial clues. As the friends begin crafting a note to their counterparts, a strange man appears outside, heightening their anxiety before he vanishes. They find the identical note they had created already affixed to their door, a chilling sign of the alternate reality.
Determined to uncover the truth, Emily, Kevin, Mike, and Laurie decide to investigate the other house. There, they encounter doppelgangers wielding red glow sticks, prompting both groups to run back home in a panic. Explaining their shocking experience, Emily’s group admits they saw versions of themselves, igniting discussions about the implications of their discovery. Hugh, armed with notes from a physics lecture, suggests the comet may have split realities, with one destined to collapse as it passes.
Tension spikes as the group debates how to handle their doubles, particularly when Mike suggests taking drastic measures against them. He proposes stealing the physics notes from their alternate Hugh to ensure their other selves don’t gain the same knowledge. The dynamics shift again when it's revealed that Hugh and Amir are actually from the alternate house. After some back and forth, the Hugh and Amir with blue glow sticks return, bringing more shocking revelations about the existence of additional realities.
As chaos ensues and accusations fly—including Beth being wrongfully accused of drugging the food—the friends face an escalating crisis. They hide outdoors only to find someone shattering Hugh’s car window, prompting further investigation. In a moment of vulnerability, Emily shares a heartfelt conversation with what she believes to be her Kevin, only to make a jarring discovery: she’s speaking to a version of him from a different reality.
Determined to find a solution, the friends decide to leave a marker in front of their house—an object and photographs with numbers from dice rolls. They soon realize discrepancies in their memories and numbers, leading to confusion and paranoia about their identities and experiences. Emily finds herself at odds with various sets of memories, discovering that only Lee and Beth’s recollections align with one specific group of numbers.
As the drama unfolds, a blackmail note exacerbates the already tense atmosphere. Mike confronts the possibility that a darker version of their group might exist, intensifying the fear within the house. Then, an ominous new version of Mike arrives with malicious intent, causing panic among the group. In a moment of desperation, Emily seeks escape to other realities, some of which reveal even graver situations.
Ultimately, Emily stumbles upon a version of herself that is blissfully unaware of the turmoil—where she and Kevin had a strong relationship from the start. Seizing the opportunity, she hatches a plan to replace her alternate self. In a frenzied clash, she ambushes her double to prevent her from returning. Yet, in a surprising twist, the ambush takes a toll on Emily, leading her to faint.
Upon awakening the next day in a seemingly stable reality, Emily embarks on a quest to find her double but instead encounters Kevin, whose phone then rings with an incoming call from none other than Emily herself. His suspicious gaze suggests that even this newly formed reality may not be as safe as it appears, leaving the audience questioning the stability of identity and existence in the shadow of a cosmic event.
