Primer is noted for its extremely low budget, experimental plot structure, philosophical implications, and complex technical dialogue, which Carruth, a college graduate with a degree in mathematics and a former engineer, chose not to simplify for the sake of the audience.

The duo's divergent personalities – Abe cautious and controlling, Aaron impulsive and meddlesome – put subtle strain on their collaboration and friendship.

As the film progresses, the two men begin to notice alarming side effects of time travel which take the form of earbleeds.

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.

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.

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.

[6][7] Although one of the more fantastical elements of science fiction is central to the film, Carruth's goal was to portray scientific discovery in a down-to-earth and realistic manner.

[8]Carruth has said he intended the central theme of the film to be the breakdown of Abe and Aaron's relationship,[9] as a result of their inability to cope with the power afforded them by this technological advancement: First thing, I saw these guys as scientifically accomplished but ethically, morons.

The time machine itself is a plain gray box, with a distinctive electronic "hum" created by overlaying the sounds of a mechanical grinder and a car engine, rather than by using a processed digital effect.

"[10] Carruth chose to deliberately obfuscate the film's plot to mirror the complexity and confusion created by time travel.

The small budget required conservative use of the Super 16mm filmstock:[4] the carefully limited number of takes resulted in an extremely low shooting ratio of 2:1.

Dennis Lim of The Village Voice said that it was "the freshest thing the genre has seen since 2001",[18] while in The New York Times, A. O. Scott wrote that Carruth had "the skill, the guile and the seriousness to turn a creaky philosophical gimmick into a dense and troubling moral puzzle".

[19] Scott also enjoyed the film's realistic depiction of scientists at work, saying that Carruth had an "impressive feel for the odd, quiet rhythms of small-scale research and development".

[19] There was also praise for Carruth's ability to maintain high production values on a minuscule budget, with Roger Ebert declaring: "The movie never looks cheap, because every shot looks as it must look.

"[20] Ty Burr of The Boston Globe commented that "aspects of Primer are so low-rent as to evoke guffaws", but added that "the homemade feel is part of the point".

[21] Similarly, Wired wrote: "Primer was noted for its originality – the film takes on complex topics like quantum physics and doesn't dumb them down for the viewer, instead using real jargon and terms that real-life researchers would – and for its commitment to a lo-fi aesthetic.

Much of the film is set in garages and car parks, and then with the exception of the two lead roles, every other character is played by a friend or family member of the cast.

Club: "The banter is heavy on technical jargon and almost perversely short on exposition; were it not for the presence of voiceover narration, the film would be close to incomprehensible.

"[5] For the Los Angeles Times, Carina Chocano writes: "sticklers for linear storytelling are bound to be frustrated by narrative threads that start promisingly, then just sort of fall off the spool".

[17] "Primer is hopelessly confusing and grows more and more byzantine as it unravels," Chuck Klosterman writes in an essay on time travel films five years later.