Even though the main explanation is consistent and ties the whole movie together there are still a few parts of the movie that are not explained and have to rely on theory. Here are some of those questions with possible explanations and some common mistakes people make.
What Creates The Tangent Universe
Why exactly does a Tangent Universe (TU) occur? Are they a natural phenomenon or are they artificially created. There is one theory that scientists in the future build a machine that somehow causes a rip in time back in the past. Another theory is it could have been caused by Alien involvement.
Quite a common thought is Frank saving Donnie in the beginning is the cause of the TU. Donnie was meant to die and by surviving he altered things which caused the corruption. This is not true though, we are already within the TU when Frank wakes Donnie up. Frank's special powers only work within the TU so he would only be able to wake Donnie after the TU had started.
The same goes for the Artifact not being the cause of the corruption. The jet engine appears several minutes after the TU had started. The Philosophy of Time Travel states that the Artifact is the first sign of evidence that a TU has occurred.
The First Jet Engine
One major question is where does the jet engine in the beginning of the movie come from originally? If it came from the future where exactly and what day did it come from?
A common answer to this question is it's the same engine Donnie sends through the portal from the plane his mother and sister were on. The engine travels through time and lands on Donnie's house. It's exactly the same event as the end of the film only the first time round Donnie survives.
This is extremely unlikely though as the portals are just a gateway between the two Universes. The first engine lands in the Tangent Universe (TU) so if it came through a portal it would have to come from the Primary Universe (PU). The engine Donnie rips of the plane at the end is from the TU and travels through a portal into the PU so there is no way it can be the same event repeating.
If it did come from a future point in the PU once the corruption is fixed by Donnie that future event won't happen now so we will never find out exactly how it occurred.
Another theory is the Tangent Universe is a copy of the Primary Universe but as it's being formed there is a glitch or copying error. This error produces a duplicate engine that is then dumped into Donnie's bedroom. This anomaly makes the TU even more unstable and it must be sent into the PU.
Roberta Sparrow: The Philosophy Of Time Travel
Roberta Sparrow writes the book "The Philosophy Of Time Travel" which helps guide Donnie but how exactly did she know all this information? There are a few plausible explanations for this. It seems like she had a similar experience herself so either she was a Living Receiver or one of the Manipulated Dead in a previous Tangent Universe.
Both options would in theory give her all the information required. If she was a Living Receiver she would obviously have known what to do and the Manipulated Dead have a large understanding of what is happening within a Tangent Universe. It's possible she may have somehow retained some of the information from her experience once the Primary Universe started up again.
Does Donnie Have To Die
This subject is one of the most debated parts of the movie. A lot of people make the mistake of thinking Donnie waking up and cheating death in the beginning is what caused the Tangent Universe (TU) to occur. The assumption is then once the Primary Universe is restored Donnie stays in bed otherwise he risks causing another TU to start and the whole cycle will repeat. This theory is not correct though, Donnie and Frank have nothing to do with the creation of the TU.
We hear about other Living Receivers in The Philosophy of Time Travel who were killed by the Artifact they returned. If we include Donnie then it seems the Living Receiver always dies once the Primary Universe is restored. The question is what would happen if Donnie got out of bed again and survived? The moment we see Donnie laughing in bed at the end the Primary Universe is already restored. If Donnie got out of bed and dodged the second engine after that then in theory nothing would change as the corruption is already fixed.
Another important point is whether Donnie chooses to die. The Donnie we see in bed at the end is not the same Donnie from the last 28 days. How much knowledge he has of what happened in the TU is very questionable and it’s quite unlikely he knows he is about to die.
The question of destiny plays quite a big part here, Donnie was meant to die but Frank saved him. It’s possible he was chosen as the Living Receiver because his life is now expendable; he is going to die anyway. Once back in the real world it's Donnie's destiny to die.
Sorry for the stupid question, but how old was Donnie when he died?
@sigh - That's a great question! Probably the 2nd best question to ask.
To answer it, we have to ask: "What is Cherita's role?"
I've read a few comments here and the default answer for most characters is: "To assist Donnie as a Manipulated Living." <-- that's a very default answer/excuse for everything. I think Cherita is a bit more complex than that.
There is definitely a God aspect to the movie. Chertia is about the closest thing we have to an angel - or a guardian. She is an empath, in a sense. We see evidence in this when she listens in on his conversations with the science teacher, when she dances, when she happens to be there during Ms. Pomeroy's outcry, and when she is smiling at the end. I believe she transcends the TU/PU timeline and can sense what should/will happen. She isn't surprised by much and is always concentrating on Donnie. She is very concerned about Donnie fixing the TU.
The most interesting part is that she is genuinely scared of Donnie's comment you mention. That's a bit out of character for her. She's mostly un-phased by everything.
I believe she doesn't love Donnie but admires him *almost* to the point of infatuation. She knows he will save the world and is always looking out for him. When he tells her everything will be ok, she senses that he has accepted his fate and is afraid of the reality that he will die. She knows it has to happen but doesn't, in that moment, want to admit it.
Since she is a major helper, Donnie is switching roles with her by saying that. She doesn't need that. She is an angel. Angels don't need help.
"Roberta sparrow also survived 2 brushes with death; one when Donnie and his father were driving, and the other when Frank was driving and missed her and ran over Gretchen. And if Donnie hadn't died, wouldn't Gretchen still be dead? Why would Frank go back and save Donnie, if it would've just resulted in Gretchens death and himself?"
Gretchen only dies in the Tangent Universe. Everyone in the Tangent Universe (or the copies of everyone) would die if Donnie did not return the artefact. There is no way to save the Tangent Universe, only prevent it from destroying the Primary Universe as well.
"Wouldn't it just have saved more trouble if Frank just let him die in the first place?"
Both Donnie and Frank are already in the Tangent Universe at that point. The whole end of the (primary) universe problem has already been set in motion. Frank is a Manipulated Dead, only having powers in the Tangent Universe, he is just a guy in the Primary Universe (although with possible memories or dream recall) so cannot do anything to prevent the problem.
Donnie dying is irrelevant to the outcome, barring your theory of surviving receivers causing Tangent Universe, although that would simply cause additional problems, not change this one. If he does not get the artefact (Jet Engine) out of the Tangent Universe, it will implode, destroying the Primary Universe as well. That is all he has to do. It is unclear whether he chooses to die in the Primary Universe, or does not remember the Tangent Universe at that point, other than a feeling of happiness, like waking from a good dream.
"And also, his psychiatrist admitted at the end that the medication he had been taking was just a placebo. How was she aloud to do this? Without the consent of himself or his parents?"
She is Manipulated Living, likely switching his pills as to not mess him up, powerful anti-psychotics could have lowered his chances of succeeding. It is in a sense not her decision, she does not weigh the consequences for her career, she just does what steers Donnie towards returning the Artefact. Like the English teacher, both with Cellar door and also perhaps with her letting Gretchen sit where she wants to (next to Donnie).
Everyone who is not the Living Receiver (Donnie) is manipulated in some way to help him. The Manipulated Dead (Frank, Gretchen) and everyone else (Manipulated Living).
The *only* question I have left, that I can't find the answer to, is why does Cherita run away from Donnie after he tells her that line towards the end? The part where he grabs her and is like "it's all going to be ok" or something like that and she runs and drops her stuff. It's shown that she likes him, so why would she do that?
It makes perfect sense that the jet engine is from the plane carrying Donnie's mother and sister.
I've watched this movie numerous times, and I've always been left with questions. And as I was reading the comments above, I was thinking about what could have caused the rip. My main theory now, is that what caused it was Robarta Sparrow. If she was the previous reciever, perhaps she was supposed to die and cheated death, so now it's after Donnie. If it were all simply just a dream, then what was the point of all the info about time travel and Roberta Sparrow? Roberta sparrow also survived 2 brushes with death; one when Donnie and his father were driving, and the other when Frank was driving and missed her and ran over Gretchen. And if Donnie hadn't died, wouldn't Gretchen still be dead? Why would Frank go back and save Donnie, if it would've just resulted in Gretchens death and himself? Wouldn't it just have saved more trouble if Frank just let him die in the first place? And also, his psychiatrist admitted at the end that the medication he had been taking was just a placebo. How was she aloud to do this? Without the consent of himself or his parents? Just some of my thoughts. I love reading about everybody's theories.
Two things that stuck out to me that isn't on this site
1. Why does Donnie wake up on that road in the beginning of the movie? Most would probably say he slept walked or he's got mental issues but if that's so why go through the trouble of putting it in the movie? Nothing in the movie was a fill or just a scene. It's all got to mean something. They've thought up all that and they wouldn't just waste away parts of the movie.
2. I somewhat understand why Gretchen waves at the end to Donnies mom but I don't understand why after the mom waves back the neighbor kid waves to her. Now this site has done an amazing job explaining the whole picture and figuring all this out but there are things that can't be solved. The answer isn't that he thought she was waving to him cause again why put it in there then? It's gotta relate to the story somehow and through all the thinking and rewatching (this is my favorite movie) I can't find an answer to those two questions and I'm sure there are many that I will come up with or have that I just can't think of but try explaining that.
I think Dan Smith did a great job on this site. Johnny Darko is one of my favorite movies and I intuitively understood much of the plot yet much remained a mystery. This explanation enhances the movie for me especially since the theoretical science itself is the best mystery of all. I wish more movies would break from the tendency of Hollywood to follow the formulaic solution to appeal to the simplistic masses. Movies like David Benioff’s Stay, David Lynch’s Mulholland Drive, Zhang Yimou’s Hero, or more recently Yann Martel’s Life of Pi are films that transcend the mediocre and become art. Thank you Dan Smith for doing such a wonderful job on this website for those that really live the medium of film.
How the fuck does Donnie destined to die when the jet engine comes from the future?
What is in chapter 11?