The film is inspired, in a peculiar way, by Francis Parkman's travelogue of the same name, which is credited in the script with inspiring people to take the dangerous westward journey to Oregon. The filmmakers overstate their case just a little. Their film, set in 1846, opens with the aftermath of an Indian attack on a settler family. Amid the wreckage is a scorched copy of Parkman's book. The problem with this is that while Parkman had already published his narrative in serial format, The Oregon Trail wouldn't appear in book form until 1849. Parkman, who isn't a character in the film, is denounced by newspaper editor James Gordon Bennett, who perceives a greater danger on the trail to Oregon. He assigns ace reporter Neal Harris (MacMurray) to join a wagon train and investigate whether the U.S. government is infiltrating troops into Oregon for a showdown with Great Britain, which disputes the border between Oregon Territory and Canada. As it turns out, Bennett is right. President James K. Polk assigns Captain George Wayne (William Bishop, who was dead within months of the film's release) to make his way to Oregon with the very same train in which Harris is traveling. So far, so nearly the stuff of Seventies conspiracy films.
Harris and Wayne meet a variety of characters in the train, including a potential love interest for either man in Prudence Cooper (Nina Shipman), the grizzled guide Seaton (Henry Hull) and the eccentric Garrison (John Carradine), for all intents and purposes the legendary Johnny Appleseed. There's also the obnoxious Brizzard (Tex Terry), who likes to pick fights with Harris and favors a bullwhip. As Harris grows suspicious of Wayne and his sidekick, who can't help calling Wayne "Sir," the party encounters the grisly remains of the massacred family from the prologue and has to go on short water rations when a waterhole Seaton depends on finding turns out to have gone dry. Brizzard goes berserk when he sees Garrison watering his baby apple trees, assuming that the old crank is stealing water when he's actually sacrificing his own ration to keep the trees alive. Harris comes to Garrison's defense and brawls with Brizzard until a sudden rainstorm resolves the matter. The scene closes with an amusing, almost Brueghelian moment as the pioneers scramble to catch rainwater in any available basin while Harris and Brizzard, still brawling, roll obliviously through the fresh mud in and out of the frame, until Garrison finally breaks things up with a swat to Harris's rear.
For much of the film Henry Hull guides the brave pioneers through the dangers of the great outdoors (above)
and the perils of the 20th Century-Fox soundstage (below).