Donnie Darko is an intelligent, yet delirious and emotionally-ill teenager, who's idealistic fantasy of correcting the wrongs in society, finally appears possible in a 'tangent universe' (an imaginative reality), where he is guided by an imaginary friend Frank, who appears as a malevolent giant bunny in Donnie's day-time hallucinations.
In the early stages of the film, Frank literally lures Donnie from reality and introduces him to a tangent universe where Donnie observes what would've been, (yet ultimately is) his own death in reality, where he is crushed by a plummeting airliner engine that crashes through the roof of his house directly above his bedroom where he sleeps. In the tangent reality though, he is merely a bystander, bemused and frightened by the calamity of the aftermath, as he returns home the morning subsequent to the disaster. He realizes that his long sleep-walk adventure the previous night, (Summoned by Frank) had ensured the avoidance of his death.
The viewer is initially under the impression that Frank is a product of Donnie's disturbed and inventive sub-conscious, as he is but a vehicle to allow Donnie's inhibitions to express themselves through acts of desecration. In many ways, Frank seems to take advantage of Donnie's mental state by coercing him to perpetrate crime. Yet Donnie appears to advocate his own actions, indicating his intentions for societal change, reformation, and also for companionship - he fears the prospect of "dying alone," in which case, Frank is an ally who can assist Donnie in coping with his emotional struggles and hardships.
The fact that Donnie's mental condition is later classified as 'paranoid schizophrenia' by Dr Thurman (Donnie's psychiatrist), would appear as another viable theory or impression, for the viewer to follow. However, it proves not as strong. The reason being, is that the initial or aforementioned...