Grandpa has Luke prepared for the worst when it is learned that Kate's mother is coming for a visit, but everyone is surprised when she arrives and repeatedly chides Kate for being insufficiently attentive to Luke's needs.
Grandpa reluctantly agrees to let Luke and Kate trade the McCoys' family car (plus $80) for a newer model, but they regret their decision when they realize that he was genuinely attached to the ancient vehicle.
Little Luke desperately wants to join the school band as a trumpet player, but he has no talent for the instrument and Grandpa feels compelled to resort to chicanery in order to help him prevail when tryout day arrives.
When it seems that Grandpa's location of water by divining rod is a failure, Luke employs a geologist and drilling equipment as Amos gloats at the apparent unsuccessful outcome of Luke's chosen methods.
Grandpa solicits George MacMichael's help in turning the tables on two swindlers who convinced him to pay $35 for a worthless dog, but he begins to have a change of heart when he discovers that Little Luke has become quite attached to the animal.
The stakes escalate as Flora, Kate, and Hassie refuse to go to the annual Grange Dance with Grandpa, Luke, and Tommy unless they receive a proper apology for Grandpa's remarks to Tommy about how a man should "handle" women.