Leland Stanford, former Governor of California and a racehorse owner, hires Muybridge to photograph his horse, Occident, trotting at high speed. Stanford believes that at some point in the trot all four of the horse's hooves leave the ground simultaneously, a point disputed at the time.

Muybridge faces technical challenges in capturing clear images of a horse in motion using the available photographic technology. Early attempts are often blurred or inconclusive.

Muybridge discovers that his wife, Flora Stone, has been having an affair with Harry Larkyns, a drama critic and bon vivant. He confronts Larkyns and shoots him dead. Muybridge is arrested and charged with murder.

Muybridge's trial becomes a sensation. He pleads insanity due to a head injury sustained in a stagecoach accident years earlier. He is acquitted on the grounds of justifiable homicide.