Charlie Chaplin, the hero of Glen David Gold's novel, suffers the social anxiety that often troubles gifted performers. He frets about how to introduce himself at a party: "'Hello, everyone. I'm Charlie.' Was it more pretentious to leave the 'Chaplin' off? It was a Trojan horse of informality, because it pretended he was Charlie No-One-Special, when it actually relied on those to whom he was being introduced knowing the last name themselves."

Gold's Chaplin is a poignant character, the son of an alcoholic father and a mentally ill mother, with insecurity gnawing at his core. The story follows him as he plots to keep his alarming mother tucked away in an English asylum and marries 16-year-old ingenue Mildred Harris because she's unthreatening. But this entrancing novel, set against the backdrop of the First World War and the earliest days of motion pictures, is much more than a Chaplin vehicle.

Gold's first novel, Carter Beats the Devil, (2001), was a portrait of another early 20th-century celebrity, magician Charles Carter. This book has a bigger cast and a broader sense of history. Among the cast are an aspiring movie star who rescues a puppy whose acting talent eclipses his own, his father, a Buffalo Bill copycat who takes a Wild West show to Berlin on the eve of war, and an unscrupulous Girl Guide.

If you're wondering how all these adventures connect to Chaplin, the answer is that they don't  or not much. Gold's project seems to be to show how Chaplin imbues the spirit of the era. Though the story roams widely, Gold makes everywhere he takes us seem pungently real. A street car in a Russian harbour town smells "as if someone had coughed into a sardine bucket". An abject French prostitute has "a single eyebrow" and "a ditch-digger smile".

The nascent film industry is a fascinating part of the novel's backdrop. In 1914, Hollywood is so undeveloped that the streets of Beverly Hills end in bean fields and the press is actually squeamish about discussing an actress's love life. By the end of the novel, bulldozers are destroying the meadows to make way for bungalows and studios are plotting to take over movie distribution and deprive Chaplin of his creative independence.

Reading Sunnyside feels a bit like watching a historical epic with a staggering budget. You don't mind when the pace is sluggish, because Gold keeps you busy ogling the lavish costumes and spectacular sets. But he also knows when to reward you with a marvellous close-up.

Glen David Gold's first novel in the eight years since his bestselling, much-loved debut Carter Beats the Devil opens with the death of Charlie Chaplin. Not on Christmas Day 1977, as is usually reported, but on November 12 1916, drowning, of all things, in a sinking dinghy off the coast of northern California.

Or at least, that's what Leland Wheeler sees. Leland, a handsome lighthouse-keeper, spies the struggling boat in the middle of a dangerous stretch of rocks. Dumbfounded by what seems to be the Little Tramp himself in full costume, Leland sets off to rescue him, but watches in horror as the boat disappears under a wave. Up bobs "the battered black derby, with a single strand of seaweed, like a rose upon a coffin". Chaplin is dead.

Meanwhile, in Beaumont, Texas, the townsfolk gather in their finery to greet a train bringing Charlie Chaplin on a whirlwind visit; but when the train arrives, it carries only awkward, snooty Hugo Black, an overeducated young railway engineer. The citizens respond by setting fire to the train and knocking Hugo unconscious. Afterwards, no one can quite remember why they were expecting Chaplin in the first place. In hotels and private clubs all across the country, the name of Charlie Chaplin is announced and sought. He is seen everywhere, expected everywhere, in more than 800 separate locations, and yet he is nowhere, not even drowning off the coast of California.

He is, in fact, on the roof of the Los Angeles Athletic Club, where he makes his home, drinking a cup of tea, oblivious for the moment to the extraordinary mass delusion that has swept the country, a delusion that reveals for the first time the impact of the moving picture. The Little Tramp has briefly stepped off the screen in a single, shared moment of national yearning for the most famous man in the world.

It is a bravura opening sequence, purportedly true, and the perfect beginning to Gold's massive, hugely researched explication not just of Chaplin and early Hollywood, but of America's involvement in the first world war, of the French trenches and the Russian front, of wild west cowboys and Bolsheviks, of Mary Pickford, Kaiser Wilhelm, Douglas Fairbanks, Leon Trotsky, and eventually even Rin Tin Tin.

Though they never actually meet, Leland, Hugo and Chaplin make up the three strands of the story. Leland has a face made for the screen and secretly auditions for film jobs. His formidable mother, for reasons of her own, is horrified at the thought of Leland working in showbusiness and leans on him to enlist for the coming war just as he's about to have his first success. Bitterly, Leland discovers that his father was none other than Wild Duncan Cody, a poor imitator of Buffalo Bill. Duncan died a drunkard shortly after performing the world's worst wild west show for the kaiser in Berlin. Leland takes the Duncan name and ends up servicing planes in France, where one day he discovers two Alsatian puppies in a bombed-out kennel. Here, at last, is a reason for surviving the horrors of war.

Hugo Black, meanwhile, a priggish young man who even at nine was "as rigid and duty-bound as if he were 45", ends up going to the Russian front, eventually fighting under the (real) British general, Edmund Ironside. These were the largely forgotten soldiers left behind after the armistice to fight, unsuccessfully, the Bolshevik threat in the frozen Russian hinterland.

Far and away the best strand of the novel, though, is Chaplin himself. A genius, and an overwhelmingly popular one, he is nevertheless finding it harder and harder to connect with his audiences. Unhelpfully, the US government has asked Chaplin not to enlist, but doesn't allow him to make this known. White feathers begin to appear among his fanmail, and he's on the verge of despair. "The fact was, at this moment, he wanted the world to love him forever so he could tell them, forever, what idiots they were for doing so."

He finds two unlikely saviours. Avuncular secretary of the US treasury William McAdoo convinces Chaplin to take part in the Liberty Loan tours, fundraising for the war effort. Even with Chaplin's pacifist misgivings - at the biggest rally, it is Chaplin alone who meets the haunted eyes of a returned soldier - McAdoo's intervention saves him from the accusations of cowardice. And then there is the glorious Mary Pickford, Chaplin's most hated Hollywood rival. Blessed with a head of unearthly curls which conceal a startlingly sharp business brain, Pickford dances around Chaplin in the press. When she announces Hollywood's first million-dollar contract, Chaplin renegotiates his for $1,025,000. But as studio bosses close in, it looks as if only an alliance might save both their careers.

Sunnyside is a big novel in all senses: 560 dense pages, with a huge cast and an authorial tendency to pile diversion on diversion until it nearly collapses under its own weight. There are breathtaking moments here - three paragraphs on the treatment of Native American soldiers in France are so piquant and engrossing they could be a novel in themselves - but too often it feels as though Gold can't help including nearly everything that struck his fancy. He can be both immensely charming and wildly imaginative, but Sunnyside is finally too obsessive to be an entirely comfortable read. The Chaplin strand, for example, is superb, a close-up examination of the difficulties of genius, particularly for the genius's acquaintances. But it's sandwiched between two lesser stories and any number of digressions on war finances or dog training. Gold has said he cut Sunnyside from its original 1,000 pages. Perversely, I wonder if that might have been the better book, a real once-in-a-career epic which, with more room for the facts to breathe, would paradoxically move faster and therefore feel shorter. This is a novel that inspires impatience, not least because so many good ideas are fighting to get out that none of them quite gets the airing it deserves.

 Patrick Ness's The Ask and the Answer is published by Walker Books. To order Sunnyside for £16.99 with free UK p&p call Guardian book service on 0330 333 6846 or go to guardian.co.uk/bookshop