“All is Lost” opens Friday at Sundance Cinemas. PG-13, 1:46, four stars out of four.

Robert Redford says very few words in “All is Lost,” and two of them are “I’m sorry,” repeated three times in a single message delivered in voiceover at the beginning of the film.

Who is he sorry to? And for what? Although late in the film we get a sense of who that message is for, much of Redford’s character remains a mystery (including his name, listed only as Our Man in the closing credits). This is a film that rests entirely on the presence and actions of Redford’s character, not on any backstory or dialogue, to make us care about him.

And we do — it’s an extraordinary feat of pure physical acting for Redford at 77, who has been reliably good in almost everything but hasn’t been truly great in a while. “All is Lost” is disciplined, elemental filmmaking built around a disciplined, elemental performance, and it’s one of the best films of the year.

All we know about Our Man is that he’s rich enough to own a well-appointed boat, and independent enough (or lonely enough) to sail it solo around the Indian Ocean. Trouble arrives in the opening frames, as a wayward shipping container (first shot at an an angle, like a shark’s fin slicing through the water) punctures a hole in the side of his boat. It’s not a fatal blow, but it hits right where his communication equipment is housed and renders it useless.

Resourceful and knowledgeable, Our Man fiigures out how to dislodge the container, patch up the hole, and bail out his cabin. But the collision sets in motion a chain reaction of calamities and small errors that add up to a serious threat to Redford’s character. What starts out as a serious annoyance becomes an elemental fight for survival.

J.C. Chandor’s first film was “Margin Call,” a financial thriller structured around a series of two- and three-character conversations. Perhaps overcompensating a little, there’s no talk in “All Is Lost,” no other characters, just Redford and the boat and the sea. The film is relentlessly suspenseful and gripping, as Redford’s character fights alone, grimly, against what seems like an inexorable fate. The audience can’t help but put ourselves in his waterlogged shoes and wonder how we’d fare in similar circumstances. (For me, I’m sure I would have been dead several weeks before the events of the film even begin.)

Chandor beautifully captures both the beauty and the terror of being alone in the middle of nowhere, floating way off the grid, the camera sometimes whipping back and forth across the deck in the middle of a vicious storm, other times gazing serenely up at the boat from the bottom of the ocean. Alex Ebert of Edward Sharpe and the Magnetic Zeroes provides a surprisingly muted and haunting score that blends in almost inaudibly with the sounds of the sea and the creaking of the boat.

In the end, “All is Lost” is a film about an existential crisis as much as a nautical one, pointing out the folly of one man’s dogged self-reliance in the face of vast, pitiless forces. Ironic that the film’s ultimate message should be about our need to rely on other people, since Redford carries the film all on his own.