In Primer, Aaron and Abe are two engineers working in different technical projects in Aaron’s garage. One of their experiments pay off when they discover a time loop side- effect which can make an object disappear from the present or at any previous moment.
Abe makes some adjustments and builds a time machine that can accommodate an object the size of a human. Using this machine, Abe travels six hours back in time. While doing this, the Original Abe stays in a hotel room. After six hours, he goes inside the box and becomes Future-Overlap-Double Abe. This Future Abe goes to Aaron and explains everything to him as he shows him the location of the time apparatus. At the end of the sixth hour, the Original Abe enters the “box” again and disappears after rewinding the six hours that have passed. He becomes the Future Abe again afterwards.
Abe and Aaron repeat the same routine until they are able to use this to predict the outcomes of stock trading in the stock market. The difference on their personalities causes tension between the two men. This strain on their relationship intensifies when one day, Granger (Abe’s girlfriend’s father), comes and overlaps with his present self. Aaron chases him and the man becomes comatose afterwards. Aaron assumes that Granger must have entered the “box” at some time in the future and caused changes that affected time. Because of this, Abe realizes the danger of meddling with time and he decides to go back in time to prevent himself from pursuing the experiment using a “failsafe box.”
His plan disintegrates when Future Abe finds out that the Original Aaron has already been confronted with the Future Aaron who told him the truth providing evidence of recordings and old conversations.
The pair set aside their differences for a while and go back together in time to save Abe’s girlfriend from being shot by a gunman. They succeed and they become town heroes. Afterwards, the two decide to go on their separate ways. Aaron decides to try his luck abroad while Abe remains to continue his quest in sabotaging his original “box” experiment.
The story ends with the existence of multiple Aarons occupying the same timeline while the original Abe still tries to keep himself living in the present and oblivious of the future. As the movie reaches conclusion, an alternate Aaron can be seeing relaying instructions to French workers building a warehouse-sized “box.”