Chief Dan Mathews of the Highway Patrol picks up a hitchhiker on the side of the road. Only this hitchhiker is a doctor, making routine house calls in a very rural area when his car broke down.
Fred Bell, a rancher and the father of one of Doc Leonard's patients, thinks it's more urgent and calls to see why the doctor has been delayed. Foreman Marty Grower tells Fred that the kid will make it okay then hits Fred up for an advance on his future wages. After a brief argument, Fred gives Marty some more money in which Marty promises that this time, it's a loan.
Fred goes to check on his son Johnny. The boy is in bed and weak, too weak to even play games with his dad. Marty goes back to his cottage and starts arguing with his wife Doris, who doesn't give in to his tirade. She knows her husband is a gambler and a loser. While Fred is still trying to cheer up his son with stories of future fish to catch, Doc Leonard finally arrives.
As Mathews calls the Doc's garage to have his car towed, Doc Leonard examines the foot that Johnny stepped on the rusty nail with. The doc takes Fred out of the room and tells him they're in trouble. The boy's leg has gas gangrene, a rare form, and the only way to treat it is with a rare serum. Doc Leonard calls a pharmacy but even though they have it, it's been expired over a year. Dan Mathews steps in and asks where they can get the serum. Leonard says in a small city but because they are so remote, it would have to fly in and then be driven the rest of the way. Mathews says that it needs to be done the right way and they race toward the police car, with Marty bringing up the rear.
At the car, Mathews tells the doc they should be driving him to a hospital. Leonard says the move the boy would prove fatal, that the drug must be flown in and quickly. He only has between two and six hours, if that. The Chief calls headquarters giving all necessary information when Marty chimes in that the backroads would be faster. The doc goes to arrange for Mrs. Gower to stay with the boy as Marty tries to talk Mathews into letting him pick the serum up by himself. Dan says they'll both go.
Less than an hour later, a plane lands at the Mason City airport with the drug which is transferred into a waiting Highway Patrol car. Unit 3822 picks up the serum and begins the long run to meet Mathews, running Code Three (lights and siren) all the way. 40 miles later, an officer hands off the package to Mathews. Time elapsed has been about 90 minutes.
Fred Bell corners Doc Leonard as he comes out of Johnny's room and wants to know the truth...is Johnny worse. But before the doctor can answer, Mathews stumbles into the house, bloodies, disheveled, and disoriented. He haltingly asks if Marty was here. Fred says no and asks where the serum is. Mathews says the serum is gone and so is Marty, as the doc helps the injured officer to a couch.
After Doc Leonard treats the concussion on Mathews head, Dan tells them all what happened. As they were driving quickly on the back roads, Marty suddenly grabbing the steering wheel and yanked the car deliberately to the right, causing the car to crash. Mathews was knocked unconscious and when he came to, the serum was gone and so was Marty. Doris says she doesn't understand and Mathews tells her it was no accident.
As Mathews runs to the radio to call for more serum, the doc sends Fred and Doris into Johnny's room with an equally as important task, keep the young man light and positive. If he gives up, they've lost the fight. The doc goes to talk with Mathews. The Chief wants to know what chances the boy has and the doc says few. The doc wants to know what would cause Marty, normally a hard working good man, to do such a thing. Mathews names several: jealousy, greed, ransom. The doc can't believe it as Dan drives away.
Inside, Fred tells the doc that his son is taking it so well and that he is the one who is really scared. The doc says there's nothing more logical than a brave little boy and his worried father.
The Chief calls another unit, 3840, to rendezvous with him but first he has another mission. He stops in to talk with the local sheriff who calls all available men to dragnet a large section of where Marty may be. Headquarters calls Mathews with the news that the earliest the plane can land is 1:40pm. Dan says that's no good and asks for a complete make on Gower, hoping that will give them a clue where he may be.
Officer Able meets Dan at the turnoff where the accident took place. Right at that spot is a giant hole in the road. Able says if he headed into the hills, they'll never find him. The sheriff shows up and says he has ten men working that area so Dan orders Able to stay with Sheriff Cook while he goes back to the ranch.
While the doc encourages Johnny to hang on, Mathews grills Doris on where her husband would be. Doris is sure Marty wouldn't do anything wrong. He loves Fred and Johnny. Mathews points to his head and said that this is what Marty has done and leaves the house. As the rain continues to come down, Officer Able arrives saying he's got nothing and wonders at the Chief's look. An idea has suddenly hit him. Maybe Gower was trying to just get over the hill. He orders Able to look one direction as he goes another. Able walks a couple hundred feet and then yells for Dan to come quickly.
There's is Gower, passed out on the ground with the package. Mathews tells Able to get the package to the house fast. Gower comes to long enough to explain. He saw the huge rut and tried to turn the car but it was too late. He broke his arm on the dashboard and couldn't drive. Since he couldn't wake the chief up, he grabbed the package and started running for the house. But he kept passing out from the pain of the badly fractured arm. Mathews tells him no one could have tried hardly. No one.
Doc comes out of the boy's room and tells Fred that it will be rough for a few days, but his son is out of danger. The father is grateful and Chief Mathews says he should be thankful to Marty Gower. As Fred goes to see his son, Mathews offers the doctor a lift...again.