Pay attention to Rian Johnson, because he’s trying to tell his audience how to watch his deliriously effective science-fiction thriller Looper. He does it when Joe (Joseph Gordon-Levitt) — a hired killer in the year 2044 whose job it is to slay people sent back from 30 years in the future — talks to his crime lord boss, Abe (Jeff Daniels). He does it when Joe meets his future self (Bruce Willis). The conversations begin to drift toward the mechanics and contradictions of time-travel, only to have that topic shut down. “It doesn’t matter,” protests Willis’ Joe 2074 — and if you’re not willing to understand that basic truth, you’re in the wrong place. Viewers are often too anxious to pull out the Plot Scalpel when dealing with films about time travel, ready to peel away every layer in search of the thing that doesn’t click. But the most compelling time-travel films aren’t about the mechanics. That’s the territory Johnson is exploring in Looper — and he comes tantalizingly close to a nearly perfect case study in how to do it. That’s not to say he doesn’t have some fun with the weird possibilities of causality. The entire concept that gives the film its name finds the assassins understanding that one of the hooded figures they’ll blow away will eventually be their own future self (“closing the loop”). The tug of war between the personalities of the two Joes is at the center of Looper, and if there’s anywhere that Johnson seems to fall short, it’s giving us enough of a context for the person Joe becomes and the relationship that he believes saves his life. Unlike many films set in the future, the details of the era play only a cursory background role; we see a highly addictive drug taken in the form of eye drops, and a cityscape where streets are dotted with encampments of the homeless. Spend time if you must on whether any particular convoluted plot point stands up to intense scrutiny. That’s how you’ll miss the way fantastical conceits can reveal remarkable ideas about changing the world by changing ourselves.