“Around alone.”

Even armchair sailors can feel the queasy fear those two words summon up. Sailing solo around the world is one of the great tests the world still offers the adventurous. Because there are no lifesaving Sherpa guides in the middle of the ocean.

“Around Alone” refers, typically, to the solo sailboat race that has gone under many sponsor names over the decades. The London Sunday Times sponsored the first back in 1968, and thanks to Donald Crowhurst, it stands out as the most notorious.

Crowhurst was a weekend sailor most at home taking his wife and three children out on coastal jaunts on their 20 foot day sailor/catboat in Teignmouth, England. But the tinkerer, inventor and entrepreneur talked himself into entering that first “Around Alone,” poetically asserted the added romance of his “amateur” status and set out — late — in a trimaran (three hulls) of his own design.

And once at sea, he realized how out of his depth (literally) he was.

“The Mercy” is a beautifully-mounted telling of the Crowhurst story, luring Oscar winners Colin Firth and Rachel Weisz into playing the Crowhursts, Donald and Clare, and the formidable David Thewlis as Rodney Hallworth, the publicist retained by Crowhurst who made him famous long before he became infamous.

Director James Marsh (“The Theory of Everything”) and screenwriter Scott Z. Burns (“Contagion,” “The Informant”) give this story a grave foreboding with sprinklings of English pluck, forbearing and wit. And Firth, Weisz and Thewlis give it heart and pathos and remind you why each is heralded as among the finest crop of actors the U.K. has ever produced.

We meet Crowhurst as he’s hawking his electronic direction finder at a boat show in the pre-GPS 1960s. He may not have many buyers for the ingenious Navicat, disappointing his two sons, who idolize him. But he himself is inspired by a speech given by Sir Francis Chichester (Simon McBurney, a terrific cameo). Chichester was made a Knight of the Realm for becoming the first official “around alone” sailor, a man who made but one stop along the way as he circled the Earth.

“A man alone on a boat is more alone than any man alive,” he says, reminding us of the poetic turns of phrase a different generation could manage. “To only do what has been done before is to live in the shadow of other men.”

He’s there to announce The Sunday Times solo sailboat challenge, a race doing basically what he accomplished, but with no stops and for a £5000 prize. No, Sir Francis won’t take part.

“Wild horses wouldn’t drag me back to the sinister Southern Ocean,” he says. “The waves there are not measured in inches and feet, but in increments of fear.”

Crowhurst hears “the siren call of the sea,” talks his local travel-trailer dealer (Ken Stott of “The Hobbit” movies) into underwriting him, designs a boat he figures will be the fastest and hires a former reporter now “press agent” (Thewlis) to drum up more sponsors and interest from the British press and the BBC.

You’re a dreamer, he’s told. “Dreams are the seeds of action,” he pontificates.

You’re not really a sailor, another suggests. “It seems to me the act of sailing makes one a sailor.”

Firth’s Crowhurst is impulsive, mercurial, a bit of a blowhard, but endlessly quotable. It’s no wonder the press fell in love with him, “the amateur” taking on this insanely dangerous quest.

Weisz, as subtle an actress as the cinema has ever produced, gets across Clare’s pragmatism and stiff-upper-lip stoicism more with her eyes than with her words. She hides her shock at Donald’s abrupt announcement to friends about entering the race, and swallows her growing fear about what is facing him, and what will become of her and their three children while he is away, assuming he comes back safe.

There’s a lot of wonderful detail that underlies the dread that the Crowhursts, husband and wife, begin to show as deadlines pass, Donald doubles down on his gamble-with-his-life-and-life-savings bet and compromises are made to get this amateur-designed two-masted/three-hulled boat ready.

Firth lets us see the pressure starting to overwhelm Crowhurst before the Teignmouth Electron, his boat, ever hits the water. There are canned soup, rum and beer sponsors, his main investor to placate, his pressure-building press agent to please.

And the boat “just isn’t ready.” He tries to back out, and for the first and certainly not the last time, realizes he cannot.

Firth’s tentative first steps, as Crowhurst, on a floating boat he hasn’t had time to test and familiarize himself with will alarm even a non-sailor watching “The Mercy.” He has no sea legs.

The chaos that greets him below (unfinished wiring and uninstalled gear, food and alcohol stored in piles) would depress anyone.

And the BBC and the whole damned town turn out for his departure. Pressure? No, none at all.

The bulk of “The Mercy” is necessarily set aboard a small boat, just Firth below decks, stowing and then dashing for the companionway hatch, seasick — or above deck, bailing out this leaky barge which doesn’t sail as he’d anticipated and takes on water in a way that tells him the aptly-named “Roaring 40s” (the Southern Ocean) are no place for this “Electron.”

He makes radio calls, detailing his depressingly slow progress, shoots a little film, and records tapes for the BBC — “Everything on this boat is wet. Not damp. Wet.” A man alone at sea “explores his weaknesses with a penetration very few other occupations can manage.”

We see that Crowhurst is competent but overwhelmed, and Firth gives us glimpses of the grim realizations he comes to. He’s brave enough to climb his mast to attempt a repair in mid ocean (daunting as hell). But he’s figuring out that neither he nor his boat can make it around the world. He cannot proceed, and he can’t go back.

“Honor” is never spoken aloud, but it figures into what we see him thinking. Thewlis, as the skeptical Hallworth, justifies taking the job of promoting Crowhurst by declaring that he “sees a part of England that has been lost, the intrepid part” that Churchill and other heroes of the realm had. Crowhurst’s celebrity and the expectations he and Hallworth have ginned up mean he cannot afford to quit, cannot survive any further South and cannot save face in any way he can imagine. Until he starts to consider cheating. Surely all that booze on board hastened that decision.

Marsh’s film lets us see the magical solitude of sailing, surrounded by dolphins or whales, the peril of sailing bare-poles (sails down) during a mid-sea tempest. The sea passages are intercut with flashbacks, mainly snippets of Crowhurst or his wife giving BBC interviews before departure. The most beautiful moment might be a hallucination, Crowhurst remembering explaining to his children what “The Horse Latitudes” are, and visualizing the horror that pretty phrase hides.

Firth doesn’t overdo the whole “going mad alone at sea” thing, letting his growing hair and beard and sad eyes tell us the calculations going on inside the man’s head.

Marsh gives it just enough scale to be an intimate epic, but Burns’ script gives the players the latitude to make something memorable out of a tale many of us already know the ending of.

It’s an old fashioned story, perhaps too conventional for some tastes. But writing this review from the cabin of my cruising sailboat, I have to confess that I loved “The Mercy.” It’s one of those limited-release films that few will see, with acting so compact and contained that everyone who loves great screen acting should.

Weisz, Firth and Thewlis give us understated, unfussy performances that lift “The Mercy,” and make it a wonderfully tragic story with a hint of magnificence about it.

MPAA Rating: unrated

Cast: Colin Firth, Rachel Weisz, David Thewlis

Credits:Directed by James Marsh, script by Scott Z. Burns. A Lionsgate/BBC Films release.

Running time: 1:42