Arthur Morgan is having a contract dispute with young contractor Brian Meeker. Morgan says the job should have been done in 5 days and refuses to pay him for more than that. They have words, Morgan slaps Meeker. Meeker punches Morgan and knocks him unconscious. Meeker then flees the scene.
Immediately afterwards, Morgan's foreman, Rudolph Preston, drives up to the house. He saw Meeker leave and sees that Morgan has been knocked out, and that the safe is open with twelve thousand dollars in it. He cleans out the safe and puts the money in his trunk. When he returns, Morgan is regaining consciousness and realizes he's been robbed. Just as Preston wants, Morgan accuses Meeker of robbing him while he was unconscious. At first Dan Mathews takes matters at face value, but a pair of glasses and a broken watch have him rethinking matters once he talks to Meeker.
Besides the obvious lesson that old men with glass jaws shouldn't slap strong young men, it also teaches that first impressions may be wrong.
By the way, twelve thousand dollars in 1958 is worth about 135 thousand dollars today, just to put the amount of temptation in perspective. It would have been nice to maybe know WHY Preston risked his job, his freedom and his reputation? Was it greed or did he have some personal wish he hoped to fullfill? Highway Patrol usually doesn't have time for such details.