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Here’s Why the Plot of ‘Happy Death Day 2U’ Makes No Sense

The ”Happy Death Day“ sequel’s attempt to go all sci-fi and explain how its time loop works falls apart at every turn

(Major spoilers ahead for all of “Happy Death Day 2U”)

“Happy Death Day 2U” is a fascinating sequel, and surprisingly ambitious for a horror follow-up. Not content simply to do another death-based time loop, it wants to try new things and delve into science fiction for real. But it’s also frustrating because it doesn’t really commit to this new direction.

When it comes to putting things like quantum mechanics and time travel in your movie, all that really matters is that they be internally coherent. Like, it doesn’t matter how I think time travel or dimension hopping works, because those things are not actually real. What matters is that the movie sets some rules and then lives by them. Otherwise your story just won’t work.

Take “Back to the Future,” for example. That series has rules that are sorta silly (specifically the idea that you can change the past so significantly that you erase yourself from existence), but that movie works because it establishes the rules and sticks with them. It doesn’t cheat for the sake of a story beat — it set rules that work for the story it was telling.

The first “Happy Death Day” kept things very simple. There was no explanation for the time loop, which is fine. The reason for the loop simply didn’t matter, as the mechanics of it were intentionally de-emphasized. This whole thing was just a fun twist on the slasher genre.

“Happy Death Day 2U,” on the other hand, actually does explain the time loop, and throws in parallel universes for good measure. But it doesn’t really set any rules or operate with consistency around its newly-central sci-fi concept. Instead it continues to treat the mechanics of its gimmick as flippantly as it did when there was no explanation. And that’s a problem, because once you try to explain something you have to go all the way.

“Happy Death Day 2U” opens with a new murder loop. The day after the events of the original movie, Ryan (Phi Vu) is murdered and immediately finds himself in the same situation Tree (Jessica Rothe) was in last time around. Why? Who knows? And who cares? If the hook for a “Happy Death Day” sequel was simply that it’s happening again, to another person adjacent to the previous occurrence, with no explanation, that would be fine. Not knowing why this was happening worked back then, and it would work now.

But instead of going that route, writer/director Christopher Landon introduced “Sissy,” a quantum device of some kind that Ryan built as his thesis project. Sissy seemingly turned itself on the night that Tree’s loop of murder began, and the characters quickly deduce that that’s what sparked the events of the first movie.

But no one bothers to try to explain how that would start an additional time loop a day later, when Sissy did not power on. “Happy Death Day 2U” firmly establishes that Sissy only turned on one time. So they really were only providing an explanation for the first time loop, not this new one.

So the person who is out to kill Ryan is, uh, another Ryan. Our heroes manage to capture him and interrogate him a little bit — and this new Ryan yells a lot about how Sissy is dangerous and is causing bad things to happen. And I guess he’s trying to kill this other version of himself to prevent him from using the machine to do whatever horrible thing this Other Ryan is afraid of.

So in response to this, the original Ryan fires up Sissy once again, causing an explosion that hurtles Tree into an alternate universe where she’s once again stuck in a time loop. And, presumably, causing whatever bad thing the Other Ryan was worried about and trying to prevent.

But here’s the thing: that Ryan doppelganger is never even mentioned again, nor is whatever bad thing Sissy allegedly caused. In fact, the entire opening segment of the movie is completely forgotten. Tree travels to an alternate universe but there aren’t two Trees there, just her. The implication being that her consciousness traveled, but not her physical body.

The rest of the movie is just about Tree and friends trying to use the machine to make the time loops stop and return her to her home universe, as if the inciting incident in the plot had not occurred. There’s no doppelgangers, no explanation for why Ryan would think travelling to an alternate dimension to kill his other self would help, not even a hint as to how Ryan was able to do that.

In other words: everything that the opening segment of the movie established about how this whole thing works is completely ignored the rest of the movie. “Happy Death Day 2U” spends the first chunk of its runtime kinda/sorta establishing some rules for how this stuff mechanically functions, and then immediately abandons those rules and ignores all of those story beats. Ryan’s concerns are never mentioned again, much less addressed.

From there, the vibe of the movie is very much just “fire up the machine and it will affect Tree only.” Why does it affect her in the first place? She was not near Sissy when it went off the first time and caused the time loop. And the movie does demonstrate that it can affect others, given the whole Ryan doppelganger situation that the movie conveniently forgets happened.

More importantly, why would turning on Sissy affect only Tree? Why would none of the half-dozen people present when the machine went off not be impacted in some way? And how could they know that firing it up again in the alternate dimension would not have consequences for anyone else? Again, I’m remembering doppelganger Ryan’s warning, and the movie is not.

Meanwhile, the emotional hook of the movie is that Tree has to decide between staying in this new dimension, in which her mother is alive but Carter (Israel Broussard) is dating her best friend, or returning to her old one where her mother is dead but she is dating Carter. There are a ton of potential metaphysical ramifications to this decision that no one bothers to broach. Such as, what would happen to the Tree who inhabited this new universe if the other Tree stayed? Would this be a sort of body swap situation? We don’t really get a chance to even infer an answer to this question because when Tree does return to her home dimension it appears that she returns to the exact moment she left. So nobody is the wiser, and as far as her friends are concerned nothing even happened when they fired up Sissy again.

The closest thing we get to an explanation for what happens at the climax of the movie is earlier when the Ryan of this new universe says that what they plan to do is turn on Sissy using a formula that will close the time loop in both dimensions — presumably one of these loops is the one Ryan is in. This explanation does not address the doppelganger specifically.

I can speculate a solution to this — though it’s one I have to infer because the movie does not actually provide evidence of this. My thought is that the doppelganger Ryan who tried to murder the original Ryan was the Ryan from this second universe — the one we spend most of the movie in. So we’ll say Ryan 1 is the one from the original movie and the beginning of “Happy Death Day 2U,” and Ryan 2 is the one who tried to kill Ryan 1. Ryan 2, in my theory, is the one from the dimension Tree travels to.

Ryan 2’s plan to murder himself in a different dimension is derailed by Tree’s arrival and her new death loop. Whatever plan Ryan 2 had concocted never happened because he was trying to solve Tree’s problem instead. So, essentially, whatever bad thing Ryan 2 was trying to prevent by murdering Ryan 1 also never happened — using Sissy to close the loops and send Tree home ended all the drama, and returned Ryan 2 to his home dimension or simply erased him. Basically: turning on Sissy again undid everything that happened in the whole movie.

But, again, that’s just a guess. The movie does not tell us any of that. I’m just trying to make sense of it the best I can. But, unfortunately, this guess can’t account for what happens at the very end.

See, “Happy Death Day 2U” has a bonus mid-credits scene in which all the main characters are rounded up by DARPA, which confiscated the machine but couldn’t figure out how to use it. And they want them to help by, yes, intentionally putting someone in a time loop. I can roll with the inference that with Tree’s memory of all those formulas that they can instigate something like that. But what does not follow is the idea that they could force a specific person of their choosing into a time loop on purpose.

At no point in the film do they indicate the machine’s interaction with Tree is anything other than a freak accident. All we can infer is that it “bonded” with her for unknown reasons. And it’s likely that Ryan 2 knew how to make it target a specific person because he made himself travel between dimensions using it. But Ryan 2’s knowledge would have been wiped when the good guys ended all the time loops and sent Tree home. Ryan 2, for all intents and purposes, never existed.

And, yeah, the mid-credits scene is clearly just a gag that isn’t intended to be analysed like this. Nor is the rest of the movie. But, hey, these are the waters you wade into when you try to tell a story like this one. If none of this bothers you, great! I’m glad you could enjoy the movie. But that doesn’t change the fact that this stuff is a mess. And it’s a shame, because it’s an otherwise enjoyable and amusing flick. It’s just also one that doesn’t really think through its premise.