NOTE: The nature of the film, and it's often subtle and understated approach to the subject matter, means that some of the information discussed below is established by inference rather than directly seen or explained.
Two young engineers, Aaron and Abe, work on various start-up tech projects out of Aaron's home garage, following a vaguely-discussed incident wherein an innovation they worked on was stolen by a colleague, who profited at their expense. During one such project, involving the electromagnetic reduction of an object's weight, the two accidently discover an unforeseen side-effect; an 'A-to-B' time loop side-effect, wherein an object left in the weight-reducing field exhibits temporal anomalies, proceeding normally (from time 'A,' when the field was activated, to time 'B,' when the field is powered off), then backwards (from 'B' back to 'A') in a continuously repeating sequence, such that objects can leave the field in the present, or at some previous point.
Abe theorizes that this could be the basis for a form of time travel, eventually constructing a device — the "Box" — which is large enough for a human to occupy, although not without requiring an oxygen mask so as to avoid any kind of external contamination. He conducts an experiment; switching the Box on, he then removes himself from the area and spends six hours in a room at a nearby hotel, ensuring that he has no effect on the outside world during that six hours. Once the time is up, he then enters the Box and spends six hours inside, emerging to find that he has travelled back in time six hours to the point that the Box was switched on; he has become the world's first time-traveller.
Abe meets Aaron in a public square and reveals what he has done, delivering proof by taking Aaron to the storage facility where the Box is contained to show him Past-Abe entering it to begin his journey back through time. Abe and Aaron proceed to use the Box as a device to make money by playing the stock market; discovering how the day's trading went during their six hour isolation, they use their knowledge when back in time to make lucrative same-day trades. However, the Box also begins to put a strain on their friendship, as Aaron becomes interested in exploring and exploiting the further potential of the Box, while Abe begins to grow cautious and concerned over the effects of repeated time travel. Both men also begin to experience physical side-effects following their repeated time-travelling, including spontaneous bleeding and a degeneration in their ability to write.
Matters come to a head during one time travel cycle; Aaron's increasing carelessness becomes apparent when he unthinkingly receives a phone call from his wife while in seclusion in the hotel room. Although Aaron intends to leave his phone behind when in the Box to ensure that his past self receives the call, he accidently brings it along with him, resulting in the phone call being diverted to the phone of the Aaron who has travelled back through time. This breaks symmetry and causes a paradox; although there are no immediate repercussions, the incident further unsettles Abe, who is increasingly convinced that their time travelling is a dangerous mistake. To resolve his concerns, Abe eventually persuades Aaron to join him in intentionally creating a larger paradox, by first punching a hated business rival and then preventing themselves from doing so, and then preventing some boys who wake up Abe early one morning by messing with car alarms. Before they can, however, Aaron and Abe come across another version of Mr. Granger, the father of Abe's girlfriend Rachel, who exists concurrently with the original — and who appears to be suffering the extreme effects of repeated time travel.
Abe decides that time travel is too dangerous to permit, and resolves to prevent the original experiment from occurring. It is revealed that, prior to his original experiment, he secretly built a "failsafe" Box in a secret location that could be used in the event of calamity to go back to a point before the original experiment took place. He uses it to go back in time and prevent his past self from ever travelling back in time by rendering him unconscious, then goes back to meet Aaron at the public square to dissuade him from pursuing time travel — only for Aaron to somehow predict Abe's original contributions to their conversation. At some point, a future version of Aaron has discovered the failsafe Box and used it himself to travel back in time, construct failsafe Boxes of his own, and replace his original. Overwhelmed by stress, fatigue and the confusion of the overlapping and conflicting attempts at changing the past, Abe collapses.
The two men pause to take stock of events, and determine that the chaotic interruptions to the timeline all stem from an incident at a party involving Rachel and her obsessive ex-boyfriend which Aaron intervened in; Granger was presumably attempting to prevent some serious harm which came to Rachel as a consequence. The two travel back in time repeatedly to learn all they can about what happened, and ensure that Aaron can intervene once again to ensure that Rachel's ex-boyfriend is more decisively taken out of the picture. Once this is done, and the past Aaron and Abe have been prevented from conducting the original experiment, the future Abe and Aaron — their friendship shattered, and now forced to live as both remnants of a future which no longer exists and paradoxical doubles in a timeline where they do not belong — decide to part ways. Aaron has decided to travel elsewhere to explore the potentials of time travel, while Abe decides to secretly remain in town and monitor the original Aaron and Abe to ensure that they never conduct the original time-travel experiment. But an unknown number of future Aarons may also be in this timeline, interfering for their own benefit, and it is possible that the original Aaron has secretly gained knowledge of the events that transpired from one of them. The film ends with Aaron, or at least one version of him, in an unknown country supervising the construction of a warehouse-sized version of the Box.