If you want to start with a bang, this is tough to beat. Curator Eliza Rathbone, formerly of the National Gallery of Art and the Phillips Collection and now a full-time Gloucesterite, delivers the necessary fireworks right up front. One expansive gallery here is a smoldering realm of painterly wonder: One wall is given to three mature Rothko paintings, uncanny in their inscrutable splendor; another, to Gottlieb at his peak, with his jagged primal forms swiped urgently on the canvas; and another, to Avery, the elder, brushing aside the abstract urges of his young charges with an impish tease.

I feel confident saying it’s one of the best rooms you’ll see in an American museum this summer, but more on that in a bit. This is the destination, the trio fully formed; the journey, meanwhile, awaits. In the entry hall, Rathbone grounds her subjects in the abiding friendship that underpinned their artistic communion: Avery’s warmhearted portrait of Rothko smoking his pipe, Gottlieb’s sketch of Avery at his easel.

It sets the scene with genuine affection and an appreciation for their precious shared moments. A 1957 Avery painting of stubby gulls keening over the marshy beaches of the North Shore hints at the seaside idyll; Rothko’s loving watercolors of a woman nuzzling the small child folded into her lap tips to familial bliss, and maybe something more: The softness of form, the seepage of paint, signals of the catalytic era yet to come.

To be fair, hindsight is a pretty easy read. These little Rothkos, painted in the early 1930s right here in Gloucester, speak less to clairvoyance and more to the artist’s adoration of Modern masters Henri Matisse and Paul Cezanne, who he aped but couldn’t quite emulate (Rothko, by his own admission, struggled with the fiddlier parts of figure painting, like hands and faces).

But nothing comes from nothing, and this show is here to make that much plain. Cross the hallway from the cathedral-like setting of the trio at their peak, and you’ll find a stroll through the artists’ formative years. Avery takes the starring role here; he was the Gloucester devotee who lured his young charges north from New York, where they’d met at the Art Students League.

Avery was a misfit in a great many ways, all of them finally adding up in recent years to one of the most important and unique careers in American art. For him, Gloucester had lineage: From Luminist forefather Fitz Henry Lane to Winslow Homer to John Sloane, Stuart Davis, and Marsden Hartley, its rocky shores and unearthly northern light had inspired generations of artists.

His little paintings of boats and harbors are a marvel, a clever fusion of the blooming Modern sensibility of the time and the local maritime heritage: The shadowy ruptures of “Untitled (Man & Nets),” which has no date, brims with an exuberance for color and form, almost disconnected — but not quite — from the dock and black water it so loosely captures. “Gloucester Dawn,” 1921, swims in a haze of yellow morning heat bouncing off a still sea, and casting the boats in a wavering shimmer; “Gorton’s Dock,” 1931-34, piles up the rough shapes of mast and sail, pier, shack, and hull in a mound of hectic form.

When Avery started spending summers there, artists were everywhere, though fewer of the Modern cast than he favored. He decided to bring his own; he extended the invitation to Gottlieb and Rothko — urbane, serious, intellectually driven — to maybe help them lighten up a little bit. It worked — one of the real pleasures of the show is a snapshot of Rothko on the title wall, lounging in the soft sands of Good Harbor Beach, an unexpected dash of casual repose — but it wasn’t a slackening of pace as much as an invigorating refresh.

In the harbor, the three come together with diverging interests: Gottlieb’s “Rockport,” 1936, captures his dismay at the Depression and the death of industry it wrought; a denuded landscape in mud-brown, its lonely, vacant shacks are ramshackle and untended. Alongside them both, Rothko was reaching for new ground: An untitled 1934 watercolor of a Gloucester wharf seems to almost dissolve into mist before your eyes — the drying racks for salted fish just a slash of bright white, the wharf house itself being swallowed by fog.

Extract whatever narrative you like from that, but it snaps together with a cannily chosen companion right next to it, another untitled piece from the same year of men tending gill net reels. It pushes that much further: With its angular slashes of color barely assembling into a coherent scene, it teases at an artist on the edge of destiny, looking down and contemplating the plunge.