Set during the summer of 1876, the novel follows William Johnson, a spoiled Yale student, as he is drawn into the real-life rivalry between two competing paleontologists and transformed by a harrowing journey through the American West.
Johnson, the elder son of a wealthy Philadelphia shipbuilder, enters Yale with no purpose beyond pleasure. At a dinner, his rival Harold Marlin, a fellow student from New York, accuses Johnson of being afraid to go west. Johnson impulsively claims he is joining Professor Othniel C. Marsh's summer fossil-hunting expedition, and Marlin bets a thousand dollars he will not follow through. Trapped by pride, Johnson accepts.
He visits Marsh at the Peabody Museum and talks his way onto the expedition by claiming to be a photographer, though he has never held a camera. Marsh, paranoid and secretive, desperately needs a photographer and accepts. When Johnson mentions he is from Philadelphia, Marsh grows agitated, associating the city with his bitter rival, Professor Edward Drinker Cope. Johnson spends weeks learning photography from Mr. Carlton Lewis, a local photographer in New Haven, mastering the demanding wet plate technique. The discipline begins to change him.
The expedition departs New York in June. On the train, Marsh warns the students about Cope, calling him a thief and liar; the two were once friends who studied together in Germany but became bitter enemies over disputes about credit for fossil discoveries. Marsh grows increasingly suspicious that Johnson is a Cope spy. In Chicago, Marsh meets with General Philip Sheridan, who is coordinating the army's campaign against the Sioux, a powerful Native nation, and obtains military support.
The party reaches Cheyenne, Wyoming Territory, a rough frontier town. Johnson falls for Lucienne, a trapeze performer, and oversleeps the next morning, discovering that Marsh has deliberately abandoned him, convinced Johnson is a spy. In the hotel dining room, Cope introduces himself: wiry, energetic, and open where Marsh is secretive. He invites Johnson to join his own expedition to the Judith River basin of northern Montana Territory, and the stranded Johnson accepts.
Cope's modest party includes fossil hunter Charles Sternberg, a tough Kansan; J.C. Isaac, deeply fearful of Indians; and three students, including Leander "Toad" Davis. In Franklin, Idaho, Cope is arrested on a false murder warrant he recognizes as Marsh's handiwork; he beats up the sheriff and flees. They reach Fort Benton, Montana Territory, on July 4 and learn that General Custer and his 7th Cavalry have been massacred at the Little Bighorn. The fort's captain warns that entering the Judith basin is suicide, but Cope refuses to turn back. When the War Department forbids civilian entry, Cope waits for the cavalry to ride out, then slips away with his party, joined by Little Wind, a guide from the Shoshoni nation, and a teamster-cook called Cookie Hill.
On the plains, the expedition endures rifle fire from unseen shooters, an attack by a drunken army captain, and a buffalo stampede that destroys much of their equipment. Little Wind finds evidence the stampede was deliberately triggered; Cope blames Marsh's agents. In the Judith badlands, Cope teaches the students to extract fossils, and they discover several new species. Johnson and Little Wind discover Marsh himself camped nearby, spying on Cope, and find the expedition's water spring poisoned. Cope invites Marsh to dinner, taunts him with a fake skull assembled from fragments, and fires into the air when Marsh sneaks back to steal it that night.
The expedition's crowning discovery comes when Johnson uncovers enormous fossilized teeth, each the size of a fist. Cope calculates the creature was seventy-five to one hundred feet long, twice the size of any known dinosaur, and names it
Brontosaurus.
As fall approaches and soldiers warn that Sitting Bull's Sioux are heading north, Cope agrees to leave. He stays at Cow Island to repack fossils while Johnson volunteers to retrieve the remaining crates, including the box marked with an X containing the
Brontosaurus teeth. Six miles out, Cookie betrays them, taking their horses at gunpoint and riding off with Little Wind. Johnson and Toad drive the wagon alone to camp, load the crates, and are attacked by Sioux warriors at dusk. Little Wind reappears, having been pursued, and joins the fight. Toad is killed by arrows, and a warrior slashes Johnson's lip with a tomahawk before Little Wind kills the attacker. Johnson drives the wagon over the badlands cliffs into darkness.
Little Wind, concealing a mortal wound, navigates the pitch-black terrain by throwing rocks to find cliff edges, then dies silently on the third night. Johnson, feverish and delirious, drives alone into the lawless mining boomtown of Deadwood Gulch, Dakota Territory, on August 31. Cope, seeing Cookie's arrow-riddled body on a riderless horse, presumes everyone dead and telegraphs Johnson's family that their son has been killed.
In Deadwood, Johnson buries Toad and Little Wind, stores his crates, and discovers he cannot leave: Bandits line the road south and mountain passes will close by November. He opens a photography studio but runs afoul of Black Dick Curry, a feared gunman, when a photograph accidentally captures a murder implicating Curry. Placed in protective custody to testify, Johnson misses his chance to leave with General Crook's cavalry column. Miss Emily Charlotte Williams, a young woman recruited by Curry to spy on Johnson, visits his room, sees the crates contain only fossils, and confesses. When two of the Curry brothers break in at night, Johnson kills one in the dark. He hides the crates beneath Little Wind's grave. Black Dick returns and challenges Johnson to a gunfight; Wyatt Earp, passing through Deadwood with his brother Morgan, coaches Johnson, who wounds Dick in the street.
Johnson flees Deadwood with Emily, the Earp brothers, and a driver. The Curry gang attacks twice on the road south; at one river crossing, Wyatt charges alone into the woods and bluffs the gang into fleeing. At Fort Laramie, Johnson encounters Marsh, who does not recognize the transformed young man and reveals he is heading to Deadwood to buy the fossils. Emily warns Johnson that Earp always intended to sell the bones to Marsh. While Earp stalls, Johnson coats ordinary rocks with rice paste to resemble fossils, switches them into the original crates, and hides the real bones. Marsh pays a thousand dollars for five crates of worthless rock and heads north. Johnson repacks the real fossils and flees toward Cheyenne.
In Cheyenne, Johnson cables his father for money, but his father, believing William dead, reports an impostor to the sheriff. Johnson is arrested for his own murder. From jail, he proves his identity with a telegram full of private family details and is released. Emily, feeling she will never fit into his Philadelphia world, departs with railroad baron Collis Huntington, who recognizes her as a Washington lobbyist named Miranda Lapham.
Johnson takes the train east with all ten crates, delivering the fossils to Cope in Philadelphia on October 8. Cope is overjoyed, especially by the
Brontosaurus teeth. Back at Yale, Johnson strides into the dining hall in his Western clothes, slams Marlin against a wall, and collects his thousand dollars. Marsh summons Johnson and demands the bones; Johnson tells him Cope has them all. On his way out, Johnson passes a small skeleton of
Eohippus, a miniature prehistoric horse, and the beautiful fossil fills him with sadness as he hurries down the hill toward campus.
A postscript records the fates of the historical figures: Cope dies penniless in 1897, credited with discovering over a thousand species. Marsh dies two years later, alone and embittered. Wyatt Earp dies in 1929 in Los Angeles. Sternberg becomes a celebrated fossil collector.