Deadline

While Sunset Carson is making the last mail run for the Pony Express before telegraph lines put the company out of business, he finds the manager murdered at one of the relay stations. Sunset is then attacked by three masked gunmen, who knock him out, steal a letter addressed to Martha Taylor, discarding the envelope, and turn all the station's horses loose. Sunset is forced to walk to the next station, and on the way, he encounters itinerant dentist P. Monroe Snodgrass and his assistant Joe, who give him a ride into Big Rock Creek. There he meets Martha and explains that her letter has been stolen. Martha tells Sunset that ever since her father was murdered several weeks before, she and her brother Tom have been trying to fulfill their father's contract to build a local section of the telegraph line but are being sabotaged. Martha speculates that the letter must have contained approval by the Western Union company for a time extension on the contract. Later, Grant Dawson, Martha's construction superintendent, accuses rancher Spade Gilbert of hiring men away from him and warns him to stop. With his Pony Express job at an end, Sunset joins up with Martha and Tom. When Sunset later becomes involved in a fistfight with Trigger one of Gilbert's men, Dawson, acting on Martha's orders, shoots another gunman, Pete, in the hand when he attempts to shoot Sunset during the fight. Sunset then checks Trigger's and Pete's arms for a scar he saw on one of the three men who earlier attacked him, but finds nothing. Doc Snodgrass and Joe then decide to join Martha's crew. Unknown to Martha and the others, Dawson is in cahoots with Gilbert, who intends to drive her out of business and reroute the telegraph lines through his property. Martha's crew continues to string lines until they run short of poles anmd order a new shipment. Sunset and Tom ride off to protect the wagon carrying the poles and arrive in time to stop Trigger and Pete from destroying the shipment. In the ensuing gunfight, the wagon driver is wounded, and his horses are frightened and run off. After a long chase, Sunset and Tom manage to stop the runaway wagon. Back at the construction camp, Sunset tells Martha that there must be a double-crosser in her crew and that he is somewhat suspicious of Dawson. Sunset then exhorts the men to work night and day to fulfill the contract. After someone sets fire to the poles, the men continue stringing wire over trees and bushes. Now suspecting Gilbert of being involved in Martha's disasters, Sunset and Tom investigate at his ranch and find a letter from Western Union to Gilbert assuring him that if the Taylors cannot complete the contract, the company would be glad to negotiate with him. They also find the letter to Martha granting her an extension and announcing that the company's representative will be arriving that same day. Gilbert surprises Sunset and Tom, but they overpower him and ride to save the Western Union man from an ambush, picking up the construction crew en route. By using the horses still stabled at the Pony Express relay stations, they are able to cover the long distance rapidly and arrive in time to stop the attack. Sunset finds Dawson among the would-be ambushers and, after a furious fistfight discovers that he has the scar on his arm, implicating him in the murder of the station manager. Later, Martha and Tom inaugurate the telegraph line and Tom announces that, as they have a new contract to extend the line south, Sunset will be the chief engineer.