Monk is helping Natalie prepare for a Christmas party when he gets a surprising phone call from Captain Stottlemeyer. Monk's father, Jack, is in a downtown jail. While passing through San Francisco in a tractor trailer, Jack was stopped for a traffic violation and brought in after he resisted arrest. Monk goes to the jail and finds himself face-to-face with his father for the first time in nearly forty years. Monk learns that Jack has been living in Texas and earning a living as a truck driver.
Monk gets Jack out of jail, and decides to join his father on the road. Jack explains why he left so many years ago, and he reveals that he has another son, Jack Junior, whom Monk has never met. Jack also tells Monk about the unusual nature of his current assignment. One of Jack's bosses, a man named Ben Glaser, suddenly got the Christmas spirit and decided to deliver toys to churches and orphanages all over the Southwest. He hired Jack to be his driver, and put him on a very tight schedule.
As they drive, Monk notices that their route seems unnecessarily circuitous. Then, when Monk and his father make their first drop-off at a church in Arizona, the gifts they deliver turn out to be complete junk. When Monk takes a look at the rest of the cargo, he is surprised to find a mere six boxes in the truck's huge trailer. Suspicious, Monk starts to ask questions around the orphanage. Concerned that Monk's snooping might cost him his job, Jack gets angry and drives off, leaving his son behind just as he did some forty years ago.
But then Jack gets some interesting news. Ben Glaser's partner, Kenneth Woods, has turned up dead. In addition, Jack realizes that Glaser has been lying to him about highway traffic conditions. Monk was right. Something fishy is going on. Jack turns back and finds Monk. Jack tells Monk about the death of Kenneth Woods, and they decide to continue on with their deliveries while they figure out what's going on.
As they drive, Jack reveals an important piece of information. The truck has a GPS tracking system that records the truck's last five thousand miles traveled. Suddenly, everything makes sense to Monk. Jack stops the truck while Monk explains what happened. Ben Glaser borrowed the truck and drove it to a meeting with Kenneth Woods. They had some sort of dispute, and Glaser killed Woods. Glaser hid the body, but he soon realized he had a problem. The GPS tracking system would show that Glaser had been to see Woods. Glaser had to erase the five thousand mile GPS record, and he had to do it quickly. So he hired Jack Monk to drive around on a wild goose chase that would erase the incriminating mileage log. Fortunately, Jack has yet to put five thousand miles on the truck; he's still a couple of miles short. Unfortunately, the truck has been stopped on a mountain, and the brakes give out! Suddenly, the truck is barreling down a steep decline, out of control!
Jack manages to safely stop the truck, but not before the truck exceeds the five thousand mile threshold. Ben Glaser appears to be off the hook, but later, when Monk and his father show up in Texas at Glaser's office, they have the police with them. It turns out that a noise Monk and Jack had been hearing throughout their trip was in fact a piece of Kenneth Woods' tie that had been stuck in the truck's engine. Given that Glaser can't deny being in the truck the day Woods was killed, the tie, which Woods was seen wearing the day he died, is enough to connect Glaser to the murder.
It's another case solved by Monk, and this time there is a reward beyond the satisfaction of putting a killer in jail. For the first time in many years, Monk can experience the joy of spending Christmas with his father.