In a peculiar household dominated by sibling rivalry, two brothers, Walter and Danny, navigate a daily existence filled with mutual loathing. Their teenage sister, Lisa, remains the unwitting target of their bickering while being left in charge by their absent father. However, the fragile balance of their home life is dramatically altered when Danny uncovers an ancient science fiction board game, Zathura, buried away in the dusty corners of the basement. As he starts playing, a warning card ominously predicts a meteor shower, which soon manifests in their living room, throwing their entire world into chaos.
What began as disbelief soon transforms into awe as the brothers realize the game has the power to warp their reality. To their sheer astonishment, their house is propelled into the boundless expanse of space. Meanwhile, Lisa, blissfully unaware and still napping, becomes frozen in time, symbolizing the game's unimaginable capabilities.
Deeply entrenched in Zathura, Walter and Danny face a series of daunting challenges that push them to their limits. They encounter a malfunctioning robot, a perilous close encounter with a star, and fierce attacks from the reptilian Zorgons. In one fateful moment, an unintended move by Danny conjures an astronaut, who takes critical action to disable the house's heat sources, all while drawing attention from the Zorgons. This resourceful astronaut lures the extraterrestrial threats away by igniting their couch and ejecting it from the house, creating a brief respite for the brothers to strategize.
As tensions rise, Walter insists the astronaut must leave, while Danny, taking a calmer stance, believes he should stay. Walter's growing annoyance leads to accusations of cheating when he observes Danny moving his piece too early. In a desperate attempt to rectify the situation, Walter pushes his game piece back only to find Zathura reacting by ejecting him from the house altogether. The quick-witted astronaut intervenes yet again, rescuing Walter from a grim fate.
Amidst this turmoil, Danny's long-awaited apology to Walter fails dramatically to ease the tension, culminating in yet another heated disagreement. Walter draws a surprise card that grants him a wish, igniting further discord. The astronaut, mindful of the escalating emotions, cautions Walter against a hasty decision fueled by rage. In a twist, Walter's harmless wish for an autographed American football leads to deeper questions about the origins of their enigmatic astronaut ally.
Unveiling a critical piece of his past, the astronaut reveals his own story of sibling rivalry that spiraled out of control during a game of Zathura with his younger brother. In a moment of anger, he had wished his brother never existed, a decision that left him alone in a relentless cycle of regret, unable to finish the game without his partner.
As the brothers digest this heart-wrenching tale, they finally set aside their differences, showing genuine brotherly love. Just then, Lisa awakens from her frozen state, inadvertently intensifying the brewing conflict. The Zorgons return, aggressively anchoring their ships to the house. As both brothers scramble to hide, Danny's desperation leads him to retrieve Zathura from one of the Zorgon ships, but not without attracting unwanted attention.
Driven by ingenuity, Walter uses an earlier drawn card to reprogram the robot, causing it to turn against the Zorgons, which sends them into a frantic retreat. With clarity restored, Walter then employs another wish card to bring back the astronaut's younger brother, who had been erased from existence.
As timelines begin to merge, the astronaut reveals himself to be an older version of Walter from an alternate reality. He commends his younger self for making a more empathetic choice, which catapults them into a joint existence as the astronaut fades away. Facing an intensified attack from the Zorgons, Danny showcases his skills and ultimately triumphs in winning the game.
With the conflict resolved, the brothers find themselves back in their familiar home, their house mysteriously positioned atop Zathura, a swirling black hole that had powered their wild adventure. Just as their father returns, blissfully ignorant of the extraordinary experience they've shared, Danny and Walter exchange looks of newfound respect and understanding. They vow to keep their incredible journey a secret, promising not to divulge their tale to anyone, including Lisa.
