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.
Mr. Drysdale's latest ploy involving duping the Clampetts into believing the annual running of the grunion fish is an invasion of the people of Grun backfires when Miss Hathaway revolts and organizes the female bank employees in protest.
The women employees at the bank rebel against Mr. Drysdale's unfair work practices and he locks them in his office. The Clampetts are caught in the middle where Jed tries to hear both sides but Granny takes action.
Jed and Jethro get help from a karate expert's geisha girls to run the mansion, while Granny, Elly, and the bank's secretaries all move into Miss Jane's small studio apartment as a headquarters for their cause.
Jethro's childhood sweetheart, Louellen, comes to visit the Clampetts, who believe that she wants to marry Jethro. Jethro panics and runs away because he thinks that marrying a hillbilly girl like Louellen would damage his image as a sophisticated international playboy. Elly May starts her new job as a secretary for Mr. Drysdale, who is nice to Miss Hathaway only when Elly May is around.
The final show of the series. Elly May's intended husband may win out in marrying her for the money. Hathaway tries expose his plan to Drysdale,Granny & Jed. Jethro may wed as well but may or may not return to California.