Life is one long insane trip. Some people just have better directions.
Why it is hard sci-fi
Loops a high-school drama through a tangent universe that is created when a jet engine falls onto Donnies bedroom in October 1988. The films internal book, The Philosophy of Time Travel, lays out a strict rule set. The tangent universe is unstable and will collapse in 28 days. A living receiver, in this case Donnie, must return the artefact to the primary universe. Every supernatural event in the film has a position in that ruleset.
Science inside it
Closed timelike curves and tangent-universe collapse as the films working physics. The water and metal channels Donnie sees are described in the book as visualisations of the time stream. The film commits to its own grammar and never violates it.
Spoiler alert
The tangent universe is undone by Donnie returning to his bed on the first night and letting the falling engine kill him. That collapses the tangent and restores the primary timeline, where Gretchen never meets him. The films emotional payload is that Donnie chooses to die so that everyone he loved in the tangent gets to live without him.