“Me and Orson Welles” is a little velvet sack of diamonds. It’s a sparkling love letter to a gigantic talent, a romance, a comedy, a drama. Above all it’s a tale of puberty, the period between childhood and adulthood for both of its title characters, and for America. The story is retro but the subject matter is familiar to indie director Richard Linklater (“School of Rock” and “Before Sunset”).

The film is set in 1937, as Welles, the boy wonder of radio plays and theater, prepares to stage “Julius Caesar.” Zac Efron, a 17-year-old actor bored to extinction with his high school drama class, plays hooky in Manhattan, meets the showman on the street and lands a bit part.

On the radio there’s news of war brewing in Europe, and Welles intends to make “Caesar” a revolutionary modern-dress commentary on fascism. The troupe’s producer frets about deadlines and funding, the cast and orchestra are frustrated by Welles’ constant revisions. But as opening night approaches, the crises energize everyone. With Welles as the whip-cracking ringmaster they work together in euphoric harmony.

The strongest source of tension is offstage, where the actors, actresses and office staff are performing a bedroom farce of their own. Claire Danes plays a secretary for the company whose intelligence, loyalty and cool elegance make her the perfect employee for the chaotic Mercury Theater. Efron courts her with the helplessness of a young man who believes he is in love but knows nothing of the practical side of courtships. Welles, a world-class philanderer, can go from an introduction to a smoothly executed pickup in under a minute. He’s a man in a hurry, after all, traveling across Manhattan in an ambulance to beat the traffic.

Linklater directs with a wonderful lightness of touch. He doesn’t labor over introducing the big cast of characters. We learn their identities and their place in the hierarchy as we go along.

Efron takes a step here from pinup status to actor, but the real story is Christian McKay, a British stage actor who has played Welles for years in one-man shows. His film debut is a triumph. At 36, he’s a bit old; Welles was a wunderkind of 22 when he set New York on its ear with “Caesar.” Nevertheless, McKay strongly resembles the young Welles and brilliantly captures the measure of the man. He sees everyone as an audience, to be seduced or taken by force.

McKay understands the rebellious, wounded orphan behind the emotionally isolated genius; Welles’ two great films, “Citizen Kane” and “The Magnificent Ambersons” are about celebrated men trying to recapture the hope, safety and security of their childhoods. From the cadences of that hypnotic voice to the lightning bolts of inspiration to the frequent look of a schoolboy pulling a naughty prank, McKay nails his performance to the back wall of the theater.

It helps to know something about history and Welles’ career to get the most from this film. When “Caesar” slays ’em, McKay crows, “How will I top this?” With the “War of the Worlds” broadcast and “Kane” just over the horizon, it’s a great inside joke for culture vultures. And when Efron walks off into what looks like a happy ending, it helps to remember how soon that faraway war on the radio would hit home.