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Primer

Two engineers, Aaron and Abe, supplement their day jobs with entrepreneurial tech projects, working out of Aaron's garage. During one such research effort involving electromagnetic reduction of objects' weight, the two men accidentally discover an 'A-to-B' causal loop side-effect: objects left in the weight-reducing field exhibit temporal anomalies, proceeding normally (from time 'A,' when the field was activated, to time 'B,' when the field is powered off), then backward (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 refines this proof-of-concept and builds a stable time-apparatus ("the box"), sized to accommodate a human subject. Abe uses this box to travel six hours into his own past—as part of this process, Abe stays in a hotel room, isolating himself from any communication with the outside world, so as not to interact or interfere with the outside world, after which he enters the box then waits inside for six hours (thus going back in time six hours). Once he exits the box, Abe travels across town, explains the proceedings to Aaron, and brings Aaron back to the self-storage facility housing the box. At the facility, they watch the earlier version of Abe enter the box.

Abe and Aaron repeat Abe's six-hour experiment multiple times over multiple days, making profitable same-day stock trades armed with foreknowledge of the market's performance. The duo's divergent personalities – Abe cautious and controlling, Aaron impulsive and meddlesome – put subtle strain on their collaboration and friendship. Additionally, the time travel is taxing on Abe and Aaron's bodies: effectively their days become 36 hours long when including the extra time afforded by the box. As the film progresses, the two men begin to notice alarming side effects of time travel which take the form of earbleeds. Later, they notice their handwriting progressively worsening.

The tension between Abe and Aaron comes to a head after a late-night encounter with Thomas Granger (father to Abe's girlfriend, Rachel), who appears inexplicably unshaven and exists in overlap with his original suburban self. Granger falls into a comatose state after being pursued by Aaron; Aaron theorizes that, at some unknown point in the future, Granger entered the "box", with timeline-altering consequences. Abe concludes that time travel is simply too dangerous and enters a secret second box (the "failsafe box", built before the experiment began and kept continuously running), traveling back four days to prevent the experiment's launch.

Cumulative competing interference wreaks havoc upon the timeline. Future-Abe sedates Original-Abe (so he will never conduct the initial time travel experiment) and meets Original-Aaron at a park bench (so as to dissuade him), but finds that Future-Aaron has gotten there first (armed with pre-recordings of the past conversations, and an unobtrusive earpiece), having brought a disassembled "third failsafe box" four days back with his own body. Future-Abe faints at this revelation, overcome by shock and fatigue.

The two men briefly and tentatively reconcile. They jointly travel back in time, experiencing and reshaping an event where Abe's girlfriend Rachel was nearly killed by a gun-wielding party crasher. After many repetitions, Aaron, forearmed with knowledge of the party's events, stops the gunman, becoming a local hero. Abe and Aaron ultimately part ways; Aaron considers a new life in foreign countries where he can tamper more broadly for personal gain, while Abe states his intent to remain in town and dissuade/sabotage the original "box" experiment. Abe warns Aaron to leave and never return.

Multiple "box-aware" versions of Aaron circulate—at least one Future-Aaron has shared his knowledge with Original-Aaron, via discussions, voice-recordings, and an unsuccessful physical altercation. Future-Abe watches over Original-Abe, going to painstaking extremes to keep him unaware of the future. An Aaron directs French-speaking workers in the construction of a warehouse-sized box.