The film is based on the short story of the same name by Ray Bradbury. It is about "time tourists" who accidentally interfere too much with the past, completely altering the present.
Plot:
The film opens in the Cretaceous, on a prehistoric hunting safari. The expedition includes Time Safari employees Travis Ryer (Edward Burns), Jenny Krase, Marcus Payne (David Oyelowo), Dr. Lucas, federal regulator Clay Derris and their wealthy guests Alicia and John Wallenbeck. The safari is set under very specific rules to avoid altering history: a volcano is observed on my chest to erupt five minutes after they arrive, their target is an Allosaurus (actually extinct in the Cretaceous) that would imminently get stuck in tar, frozen nitrogen bullets are used as ammunition to leave no trace, and the party wears self-contained suits and are told they must stay on a floating path and not touch anything. Furthermore, all the guns are tied to Travis's weapon, and will not fire until he does so.They successfully complete the expedition and return to 2055 Chicago, where Time Safari chairman Charles Hatton (Ben Kingsley) organizes a party to celebrate their success. The party is interrupted by the disgruntled Sonia Rand (Catherine McCormack), who sprays blood on Hatton and the Wallenbecks. Sonia is the inventor of the time machine software, TAMI, but received no credit due to loopholes in the contract she signed to license TAMI. She is later confronted by Travis and the two have a brief conversation in Sonia's lab, in which she explains to him the danger of altering the past using the time machine.
During another trip while escorting clients Eckels (William Armstrong) and Middleton (Corey Johnson), Ryer's gun fails as the Allosaurus rushes the party. The tourists go out of sight while he corrects the weapon in time to kill it. The party returns to their time, unaware that someone has accidentally stepped off the path. The next morning, the TV news describe a drastic rise in temperature and humidity, and Travis observes a sudden growth in plant life.
On a subsequent trip with different guests, the party finds that they have arrived too late: the Allosaurus is already dead and the volcano is only seconds away from erupting. When Travis returns to report these changes, Time Safari is immediately shut down by the government. Travis seeks out Sonia at her apartment, and she points out the arrival of the first "time wave", which drastically alters the city because of the changes in the past and fluctuation in the evolutionary tree. A swarm of beetles appears, and Travis causes an explosion while fighting them off; the two jump off the building, and are caught by the branches of a tree that has burst through the building. Sonia warns that more waves can be expected, each affecting a later stage of evolution, of which humans are the current final link.
Returning to Time Safari, the government works with the company to try and correct Middleton's mistake. Travis mistakenly appears in the American Southwest during the 18th century; he returns to 2055 as another time wave hits, leaving the city out of power and completely wrecked by dense vegetation. Sonia explains that the presence of time waves is preventing the machine from operating properly. Payne discovers that the expedition came back from the past 1.3 grams heavier, and that Hatton had shut down the costly "bio-filter" which was meant to prevent items from the past from arriving in the present. They scan their expedition clothes, and nothing turns up, meaning it was one of the tourists who stepped off. Seeing as Eckels was terrified on the hunt, they decide it must have been him. Sonia explains that the time machine can theoretically break through the waves using a wormhole to go back to the approximate time of the hunt, then slingshot to the arrival of the party. However, this would leave the time traveler only seconds to fix the timeline. The group goes to seek Eckels while Derris and Hatton stay behind. On the way Payne is attacked by a poisonous plant which leaves him delusional and partly paralysed. They then encounter the Baboonlizards, alternatively evolved dinosaur-primate hybrids, and Payne sacrifices himself to let the group escape. Sonia explains that even if the timeline is fixed, subsequent expeditions could still go awry unless their past selves can shut down Time Safari. They find Eckels, but he claims he never stepped off the path, so they go after Middleton. Middleton, also poisoned, is severely paranoid and commits suicide without telling them anything. In searching through Middleton's kit from the safari they discover a butterfly on the sole of his boot, the one he had stepped on in the past.
They decide to go back to Time Safari, though are attacked by a swarm of bat-like creatures on the way, resulting in Lucas being killed. They arrive, to find that it is flooded, making the machine inoperable, and that Hatton has died. They retrieve TAMI's hard drive to use the nearby university's particle accelerator as a time portal. As they leave, Derris is killed by Baboonlizards. The group then follows the also-flooded subway tracks to the university. As they get inside a train car, the ceiling of the tunnel gives way, flooding the car and leaving the three trapped inside. A long reptilian eel-like creature bursts through a window and devours Jenny. Sonia escapes and Travis fights it off long enough for the train's ceiling to cave in and pin down the creature. Sonia retrieves Travis and they arrive at the university, followed by a new, more simian kind of Baboonlizard, indicating that only one wave remains. Sonia is able to send Travis back in time just as the wave hits, leaving her as a catfish-like humanoid.
Travis successfully intercepts the hunting party and corrects Middleton's mistake. Before disappearing, he tells Jenny that the bio-filter has been turned off, and asks her to give the old Travis the recording of the expedition. She does, and Travis brings it to Sonia; she agrees to watch it, at his insistence that the video contains enough evidence to put an end to Time Safari.