Christopher Nolan’s “Interstellar” is the most ambitious science fiction film — maybe ever, certainly since “2001: A Space Odyssey.” Long, filled with lengthy passages of exposition and explanations of science, it takes forever to get to a killer third act.

But you will walk out of the theater with a better grasp of “relativity.” You will fear science, a little less. And you will want to hit the bathroom before settling in for its 2:49 running time.

In the not-distant future, human civilization has settled into entropy. Cities have been abandoned, billions have died, dust storms plague the survivors and humanity’s ability to feed itself is collapsing thanks to blights that wipe out the monoculture agriculture has become.

Matthew McConaughey is Cooper, once a test pilot for NASA, now turning his engineering skills to running a rural farm. He is sun beaten and weathered, raising two kids (Mackenzie Foy, wonderful, and Timothee Chalmet) with the help his late wife’s father (John Lithgow).

School teachers are underselling our potential, pushing the idea that we have devolved into a “caretaker” civilization, and tell Cooper’s kids Americans never landed on the Moon. So he’s teaching the kids self–reliance, reasoning.

“Figure it out. I’m not always going to be here to help you.”

Then events conspire to put Cooper back in touch with a cadre of scientists, led by Professor Brand (Michael Caine) and his scientist daughter (Anne Hathaway). They’ve cooked up a last-ditch effort to save humanity — not on our dying planet, but out there, in the cosmos. Cooper will pilot a mission through a wormhole to find us a new home, and Amelia Brand, Doyle (Wes Bentley), Romilly (David Gyasi) and a model of the cleverest, simplest, most practical robot ever depicted on the screen, TARS (voiced by comic Bill Irwin) will go with him.

McConaughey is well cast as the last of the space cowboys, a drawling philosopher who ponders why “we’ve forgotten who we are — explorers, pioneers.”

Hathaway has the cold-hearted scientist role to fulfill. And the robot provides a smidgen of comic relief. What could happen here, TARS?

“Nothing good.”

Nolan, co-writing the script with his brother Jonathan, references a staggering swath of sci-fi film history. “Interstellar” plays like “2001” as re imagined by M.Night Shyamalan, a bleak, harrowing tale that finds faith and hope in humanity’s persistence and ability to problem solve and improvise. It’s a marvelous mashup of sci-fi images, themes, tropes and science, referencing every film from the original “Planet of the Apes” to “2010,” Solaris” and “Sunshine” to Disney’s “The Black Hole.”

It has the pulse of Carl Sagan and the soul of Ken Burns, especially his documentary “The Dust Bowl.” Elderly people — sages — turn up in segments of interviews, remembering Earth in its most dire moments.

Nolan withholds full views of the space ships, which look like modern, high-mileage versions of the vehicle Charlton Heston crashed into a lake in “Planet of the Apes,” or less dingy boxy-affairs out of “Alien.” The director toys with the silence of space, occasionally overwhelming us with the emotional or emotionally fraught music of the Hans Zimmer score.

He takes us through a black hole in a sequence that’s a state-of-the-art updating of what Kubrick did in “2001.”

He creates a puzzle, which he bends the rules — it not space and time itself — to solve.

And he delivers a sermon without preaching, a science lecture without blame. The Earth’s a mess, but “we were not meant to die here.” Can we make that quantum leap, past politics, greed and fear of science to “reach beyond our lifespans?”

It is gorgeous to look at, and moving to experience, thanks to Hathaway, McConaughey, Caine and Jessica Chastain, who shows up in the latter third as the adult daughter whose father left her to go into space long ago.

Whatever its length and melodramatic third-act touches, “Interstellar” is a space opera truly deserving of that label, overreaching and thought-provoking, heart-tugging and pulse-pounding. It’s the sort of film that should send every other sci-fi filmmaker back to the drawing board, the way Stanley Kubrick did, a long time ago in a millennium far away.

MPAA Rating: PG-13 for some intense perilous action and brief strong language.

Cast: Matthew McConaughey, Anne Hathaway, Jessica Chastain, Michael Caine, Casey Affleck

Credits: Directed by Christopher Nolan, written by Jonathan Nolan and Christopher Nolan. A Paramount release.

Running time: 2:49