In a dystopian future, Earth faces a catastrophic extinction event, prompting the selection of 300,000 survivors to board a spaceship named The Ark. This vessel is destined for a new colony known as New Earth, located several light-years away. Among the chosen is Noah (Cody Kearsley), who cunningly stows away by impersonating a junior janitor while his pregnant girlfriend, Hayley (Kassandra Clementi), is placed in stasis as a passenger.
During the voyage, a surly mechanic named Clay (Bruce Willis) takes Noah under his wing, attempting to guide him through the challenges ahead. However, everything is not as it seems; the ship is infected by a parasitic force that has claimed the lives of two veteran mechanics, Blue (Johnny Messner) and Shady (Johann Urb). As Noah observes Clay closely, he becomes suspicious that the older mechanic is up to no good, suspecting he is constructing a bomb. Clay soon clarifies his intentions, revealing that he is actually brewing moonshine.
That night, the workers hold a spontaneous party, sharing stories about the loved ones they left behind. Noah mentions his future role as a butcher when they arrive at New Earth. The atmosphere turns dark when Ortega (Angie Pack) leaves the celebration to be with Blue, who later kills her.
As the plot unfolds, Clay uncovers that Noah's girlfriend is the daughter of Admiral Adams (Thomas Jane). Tension escalates when security officer Stanley (Timothy V. Murphy) discovers Shady's body and eventually identifies Blue as the murderer. The search for Blue intensifies, culminating in a violent confrontation where he attacks and kills during the chaos. An autopsy conducted by Chambers (Rachel Nichols) reveals the frightening nature of the parasite infecting the crew. The situation deteriorates further when Noah is arrested for being a stowaway, but Clay defends him and secures his release.
As discussions for survival ensue, the matter becomes grave when the corpses of the deceased crew members rise and attack, claiming Stanley as one of their victims. Chambers successfully incapacitates some of the infected, allowing Clay, Teek (Callan Mulvey), and Noah to make a narrow escape. However, the chaos continues as Stanley, now infected, releases more crew members from their quarters.
The survivors seek refuge in the arsenal but struggle to find a solution to combat the rampant infection. Clay suggests using a flame thrower, but it inadvertently triggers the fire suppression system, blurring their visibility. Meanwhile, Lincoln (Corey Large) joins the group just as more danger arises, leading to a frantic decision-making process involving escape pods and the security bay.
As the story unfolds over the next 84 days, the survivors hatch a plan to protect themselves and potentially save the passengers on board. Clay skillfully guides Noah through the air ducts to reach Admiral Adams, aiming to devise a plan. In a shocking twist, Teek confesses his role in unleashing the infection and reveals his radical motivations against the resettlement on New Earth.
With the Admiral on board and his security team awakened, they prepare for a counter-offensive against the infected. The tension reaches a peak when the infected break into their refuge, resulting in a fierce struggle. Just when everything seems lost, the Admiral detonates a grenade in a last-ditch effort to eliminate the infected, only for the remains to begin reassembling into a more formidable creature.
Realizing the threat is far from neutralized, Noah and the survivors rush to Clay's moonshine stash to create more powerful weapons. But time is of the essence as the infected manipulate the reactor and begin to spread their menace to the passengers. Clay bravely faces the mutant being while Noah and Hayley make a desperate dash for the escape pod.
Tragically, Chambers loses her life in the struggle, and Clay fights valiantly, but he, too, is overpowered. As the escape pod starts its launch sequence, Noah confronts the monstrosity, using his newfound knowledge to weaken it. Just when hope seems to fade, Clay contacts Noah from the bridge, initiating a self-destruct sequence for The Ark.
As Noah and Hayley escape and finally reach New Earth, a horrifying realization dawns upon Noah as he spots another human, only to find he is infected. In the distance looms a massive monster, prompting Noah to raise his weapon and declare, > "Burn 'em all."
