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There’s one essential rule of time travel: It must be internally consistent.

As far as we know, time travel isn’t real, meaning any movie, TV show, book, comic, or other world-building endeavor that features time travel is a piece of fiction, and making time travel work within that fiction is the goal. Maybe it’s multiversal branching timelines, a la the MCU? Maybe going back to the past risks changing history and fading out of existence, like in Back to the Future.

The exact nature of the rules doesn’t really matter so long as they’re consistent. If going back to the past sends a copy of you back in time alongside the original, past version of you, that should always be how it works. It would get confusing if all of a sudden the way time travel worked within your fictional world suddenly changed, and going back in time now requires that you replace your old self. That would be dumb, right?

Alas, 2012’s Men in Black 3 — which otherwise is a pretty solid entry in the Men in Black franchise — does exactly this. Let’s take a closer look at why Men in Black 3’s time travel rules are internally inconsistent.

More on Men in Black:
Could Men in Black‘s Neuralyzers Actually Exist?
How a Jewish Scholar from the Middle Ages Inspired Men in Black 3‘s Poignant Time Travel Story
Men in Black director, production designer look back on ‘hardest’ thing to pull off in sci-fi classic

Men in Black 3‘s Flimsy “Rules” of Time Travel

Men in Black 3 sees Agent J (Will Smith) travel back in time after an evil alien, Boris the Animal, travels to the past to assassinate Agent K (Tommy Lee Jones). Without K, a crucial shield that protects Earth from bad extraterrestrials was never built, and J must stop the assassination from taking place. He meets the younger version of K (Josh Brolin) in the late ‘60s and, after convincing his partner that he really is from the future, they work to foil the plots of Boris the Animal and his younger 1960s self. 

There are two Boris the Animals running around in the ‘60s — the one from the present day and the one from the past. A late twist reveals that there are also two Js running around in the ‘60s, as a little boy that J encounters ends up being young “James.” Going back in time in Men in Black 3, therefore, sends a person back in time where they’ll exist alongside whatever past version of themselves might have existed in that era. Seems simple, right?

Except, when J is fighting Boris on the Apollo 11 launch tower, he uses time travel in a different way to best the nasty alien. J charges Boris, memorizing the pattern of where the bad guy is firing his spikes, and knocks him off the tower. The fall activates J’s time-travel device and everything rewinds and he finds himself… back on top of the tower. Now armed with the knowledge of where Boris is going to fire, J is able to dodge the incoming spikes and knock Boris off the edge to his death.  

That’s not how time travel works though!!! According to Men in Black 3’s established rules, J going back in time should have resulted in two Js standing on that ledge, as his present self goes back alongside his very, very recent past self. He shouldn’t “replace” his old self. If that were the case, Boris the Animal would’ve replaced his 1960s self and J should’ve already replaced the little kid version of himself — which probably would have been horrifying. 

Men in Black 3 is now streaming on Peacock alongside Men in Black and Men in Black 2.