Shorty's been sent back to the hills, but the Clampetts still have all his girl boarders to deal with in the mansion; and Mr. Drysdale has serious trouble when Jean Bell's brothers get the mistaken idea that he has enslaved their sister.
Honest John shows up with another scheme to get the Clampett millions for himself. Convincing them he's invented machines to blow the smog away, Jed finds trying to get his money from Mr. Drysdale nearly impossible.
The Clampetts are determined to help the nation solve the smog problem. When Jed offers to give his millions to President Nixon for the cause, Mr. Drysdale resorts to some outrageous schemes to keep the money in his bank.
After Honest John explains to Jed why they couldn't visit the White House, he sells them other Washington landmarks each conveniently only $1 million. Mr. Drysdale finds out and sends Miss Jane to bring the Clampetts home.
Rev. Matthew's lookalike brother Mark, a Naval officer, shows ups to meet Elly May. The two head to the cement pond but later when Granny sees Mark in his diving gear she is convinced the water turns him into a man frog.
Ellie continues dating Navy man Mark over Granny's objections who is convinced he is part frog. While Jed tries to keep Mark from eating his daughter's cooking Granny works to find a cure for his amphibious ways.
Granny is desperate to keep Mark from being a frog so he can eventually marry Ellie. She heads to the bank in the belief that if Miss Jane will kiss the frog it will return to human. Jane refers her to psychiatrist Dr. Klingner.
Mark shows Granny and the rest of the family a film about his navy job to help her understand his frogman activities. She remains unconvinced. Mr. Drysdale is alarmed that Jed wants to invest millions in underwater farms.
Shorty returns and Granny convinces him to pretend to be Ellie's boyfriend to drive Mark away. Mr. Drysdale tells Jed that the idea of him giving millions to the ocean project has made Miss Jane mentally unstable.
Granny and Mr. Drysdale are desperate for different reasons. Granny hopes if Mark tastes Ellie's cooking he will leave. Mr. Drysdale dresses as a Revolutionary War general to keep Jed from investing in Mark's ocean project.
Mark finally convinces Granny he's all human, then sets about resigning his Navy commission to pursue oceanography research. Mr. Drysdale creates a diversion to keep Jed from funding Mark's new ambitions.