I don't know exactly what it is you need explaining... The philosophy behind it? The actual physical plot itself? Well, regardless, Donnie Darko is a spectacular "cause and effect" film. Basically, it all starts when Donnie is dragged out of bed, while in a subconscious sleep-like state, by Frank, who is implied to be a mere product of schizophrenia. Now, I think what's really interesting is, there is actually a deleted scene that reveals Donnie is not a schizo, and that he is actually being fed placebos. However, I sort of prefer they kept that out, as it allows the audience more room to breath, and analyze the film for themselves.

Now, back to where I was going. When Donnie is pulled out of bed, a jet engine falls directly on his bed, as you know. The serial codes are burned off, and there are no reports of a crashed aircraft. Anyways, feeling as though he owes Frank his life, Donnie feels obligated to do as he tells him to do, which leads to a series of whacky events. And of course, time-travel kicks in when Frank asks Donnie about it, and Donnie feels as though he is to look into it.

But here is what interesting. When you think about it, all of the bad that happens in the film (The firing of the English teacher, the arrest of the motivational speaker, the death of the girl friend, the death of Frank, etc.) is a direct result of the actions Donnie makes because of Frank. At the end of the film, it's revealed Frank is a guest at Donnie's party, who Donnie winds up killing. Coincidentally, Frank is also the boyfriend of Donnie's sister. But, the whole thing about the end, as Donnie gazes out over the mountains beside his dead girlfriend, and sees a time portal emerging in the sky, I think he seems to realize he could prevent all the pain he's created if he just gave himself up.

Then we're brought back to the very beginning of the film. Donnie is in bed, awake, and laughing--in the commentary, director Richard Kelly explains this is because Donnie is under the belief it was all just a dream. But I think in some subconscious way, Donnie still knows that he has to die. So he ignores Franks calls this time around, and lays in bed to die, thus saving the town of Middlesex from his "fear" as Patrick Swayze's character would have said. The jet engine fell off of his mother's plane in the future, fell through a time portal, and landed on Donnie's bedroom--thus the reasoning behind why no crash is reported. Donnie dies, and others are freed, for better or for worse.

In the end, it would seem as though Donnie is a Martyr.

the "Horror movie" double feature is EVIL DEAD & LAST TEMPTATION OF CHRIST, and the latter being choosen is no fucking coincidence.

1) Donnie's mother's aircraft engine comes back through a worm hole and Donnie also finds another wormhole to come back to the same time and same place in order to die? Right? strange coincidence....
2)If frank was real in the alternate universe Donnie was briefly living in until he dies, then why does he reply to Donnie "why are you wearing the human suit" when he is asked why he is wearing the rabbit suit? And first of all what is that rabbit suit all about?
3)If Donnie is supposed to some schizophrenic guy who imagines stuff, then Frank is also imaginary and the whole time travel thing too. Right? Then that contradicts the whole plot as I have described in (1). Why combine time travel plot with dream/hallucination/schizophrenia plot. This adds a lot more gratuitous confusion.

1) Yes. But is it a coincidence? That's open to debate, for sure. Remember the scene about the spheres, which reasserts the idea of destiny... It's as if Donnie was always meant to find a wormhole back to the exact time and place in which the engine would surface from a separate wormhole.

2) I think Frank's response was intended to be sarcastic, or maybe even a bit hostile as it was Donnie who put him in the position he was in. Sort of his way to retort and hint, "You'll find out in time". The significance of the suit? None, other than Frank died on Halloween, in his Halloween costume. So, it's only naturally that when his corpse, or soul, or whatever, stumbled across a wormhole (or a wormhole stumbled across his soul, if you will), he was sucked in while wearing his costume.

3)Well, it would, but like I stated previously, there is a deleted scene that reveals that Donnie is 100% sane. There would have been obvious advantages to keeping the scene in the final cut, in that it would clear up some genuine confusion, but at the same exact time, I'm convinced it may have deteriorated the quality of the film, by removing a large portion of the wide-range of space that is left available for the audience to interpret. So, Donnie is not schizophrenic. But even if he was, he would have had no idea who Frank was (he was his sisters boyfriend, but this is only revealed in a deleted scene, and it's implied throughout the film that the family has no idea she's in a relationship). Furthermore, Donnie had never seen Frank's Halloween costume. So the idea that Donnie was hallucinating Frank and his costume, only to come face to face with a reality of the same name, appearance, personality, etc... is highly unlikely. Besides, the only time Donnie saw Frank was when he took his medication. If the medication were real, he would not be hallucinating. Even if they were placebos, as revealed in the deleted scene previously referenced, he would not hallucinate, on the basis that the placebo effect is quite possibly the most potent drug reaction of all. I think, mainly, Frank made it a point, or destiny made it a point, only to appear when Donnie had taken his medication. This way, it'd be easier to convince him that he was not hallucinating, but stuck at the center of a bizarre reality.