You could spend a fair bit of time unpicking the logic of this time-travel thriller, which redefines the phrase "suicide mission" as a hitman ponders killing his older self sent back from the future.
But best save that for after the movie. Because this fresh reinventive genre-bender isn't just there to confound you with how past actions might affect the present. Or how time-travel, in the wrong hands, could lead to all sorts of unintended consequences. Though it does a pretty good job of all that.
No, while Looper presents with some engaging zen equations - if you decide not to do something because your future self told you you should, does he instantly lose the memory of having done it? - it's also a movie memorable for its characters, unpredictable storyline and how it reconfigures its influences into something fresh.
Though it starts out in a American Midwest noir underworld of 2044, where Gordon-Levitt Joe is a "Looper" - an executioner for a mob that sends its victims back from 2074 to be disposed of - it eventually becomes a late 21st century Western too, with shades of the likes of Shane as the story shifts to a farmhouse where Joe finds he must protect solo mum Emily Blunt (great, again) and her oddball young son.
That's after Joe has come face-to-face with his future self (Willis) who has been sent back to be killed.
Only Joe senior has other plans and has had a lot more practice at his chosen trade.
The scenes between Gordon-Levitt and Willis are riveting, even if they aren't exactly a Josh Brolin-Tommy Lee Jones match, despite the best efforts of the makeup department on the younger actor.
Director Johnson and Gordon-Levitt's earlier film together, Brick, mashed-up a high school movie and a pulp detective tale, the result of which was really too clever to be entertaining.
That's not the case with Looper, which, while offering plenty of idiosyncratic dialogue and retro-stylistic quirks, doesn't get in the way of its letting its characters breathe as they ponder one moral quandary after another, or its story ticking along with increasing urgency.
True, it does hit a bit of a lull when it heads out into the countryside for its final act. But that just lets Johnson rack up the tension again and let us ponder who, really, the bad guy might be in all of this.
And Looper ends up as a terrific lateral-thinking sci-fi thriller that should not be mistaken for being just another Bruce Willis movie. It's a one-off wonder and this year's answer to Source Code.
Cast: Joseph Gordon-Levitt, Bruce Willis
Director: Rian Johnson
Rating: R16 (graphic violence and offensive language)
Running time: 118mins
Verdict: Askew sci-fi action with a great central double act.