In the year 2067, Earth has been devastated by climate change and an ongoing nuclear war. Only one city in the ruins of Australia has been able to hold out against these catastrophic changes, thanks to synthetic oxygen; this oxygen is tainted and gradually causes a deadly affliction known as "The Sickness".
Ethan Whyte (Kodi Smit-McPhee) is a citizen who works as a manual laborer and who cares for his wife Xanthe (Sana'a Shaik), who is afflicted with the Sickness. One day, Ethan is called before Regina Jackson (Deborah Mailman), the Chief Technology Officer (CTO) of Chronicorp, a large corporation, who explains that the Sickness will eventually wipe out the remainder of humanity. During a test of the "Chronical", a prototype time machine that quantum physicist Richard Whyte (Aaron Glenane), Ethan's late father, had worked on before his death 20 years earlier, the scientists received a radio signal from over 400 years in the future with a message to specifically send Ethan to them. In hopes of preventing the extinction of humankind, Ethan is asked to be sent into the future. He refuses, as he resents his father for abandoning him and his mother, but Xanthe and Ethan's guardian and work colleague Jude (Ryan Kwanten) manage to change his mind.
After surviving the traumatic time displacement, Ethan finds himself in a lush rain forest, with a hand-held computer named Archie and a wrist device given to him as a child by Richard as his only surviving pieces of equipment. He finds the entrance to a bunker-like structure, and a skeleton wearing his jumpsuit, a decaying Archie and the wrist device, and with a bullet hole in its skull. Shocked by the discovery of his apparent death in the near future, and sick from eating poisonous berries, Ethan is rescued by Jude, who followed him through time after his life readings, transmitted through the Chronical, were failing. After sharing their findings, they follow Archie's directions to another, still-functional door, which leads to the Chronical lab. Ethan's wrist device is revealed to be a DNA analyzer specifically made to grant him access to the Chronical, which sets itself for automatic reactivation in four hours.
From a holographic recording left by Richard, the duo learns that the Chronical project originally entailed the reactivation of an atmospheric monitoring station which would ascertain the Earth atmosphere's breath-ability in the future and then transmit the data back to the past. When first activating the machine, Richard was surprised to receive a message to send his own son to the future, so despite his misgivings, he prepared Ethan's wrist analyzer. However, an immediate follow-up mission was rendered impossible because safely sending living matter through time required an operational link from both sides and the data showed a power failure in the year 2474.
The Chronical's activation triggers a malfunction in its nuclear power core, threatening to unleash a nuclear explosion before the countdown is completed. Ethan and Jude make their way to the power core, which is located beneath the overgrown ruins of their home city. Finding the ruins littered with skeletons, including Xanthe's, they conclude that a cure against the Sickness was never found. When Jude attempts to comfort him, Ethan recognizes Jude's voice from a recording he found on the decaying Archie, taken moments before his future self was killed. Claiming that he's saving Ethan from himself, Jude directs Ethan at gunpoint to the reactor's control room. Unable to activate the emergency override, Ethan decides to go inside of airlock and pull the lever.
With 37 minutes to spare, the duo return to the Chronical lab, where Ethan finds another exit that opens the entryway next to his skeleton. Ethan suffers a nervous breakdown and implores Jude to kill him, which Jude refuses to do. Jude then confesses that there was no actual hope of ever changing the future. Refusing to believe that, Ethan locks Jude in a room and plays back his father's log from the day that Richard died. Ethan learns that his mission was a sham from the beginning: Regina Jackson intended to flee from her dying time into the future with a "chosen few", while Richard maintained hope for humankind. To prevent its abuse, Richard keyed the time machine to Ethan's DNA, but when Richard's colleague announced that the machine could be rigged to send a person into the future one-way, Jackson killed Richard. Jude was appointed as Ethan's guardian to ensure that Ethan would be sent forward in time to repair the power failure and stabilize the time portal; once Ethan returned to 2067, Jackson would have him killed.
Ethan tries to shut the Chronical down, but Jude moves to stop him. When Ethan refuses to fight him, Jude, guilt-ridden, commits suicide. Just before Jackson can put her plans in motion, Ethan sends the "Send Ethan Whyte" message into the past along with a copy of Richard's recorded murder on Archie, hundreds of live jungle plants, and a farewell gift to Xanthe. He destroys the Chronical, which changes the time-line: In the past, Jackson is arrested by the police after Archie transmits the recording to a news station, and the plants are used to revitalize the planet.
In the future, Ethan's corpse is gone, and Ethan discovers his formerly ruined city 400 years in the future is advanced with architecture that is more harmonious with the natural environment. However, any future humans do not appear to be shown in the city.
