He was Mr. Krabs' neighbor during his childhood. Krabs and Plankton have him try their Plab Patty and it knocks him out. The event is placed in the newspaper and it caused an argument that ended Mr. Krabs and Plankton's friendship.
While SpongeBob and Patrick are looking for more information on the main drain, he pulls them into his house when he hears them talking about it. He says the main drain is real but unfortunately, he has no idea where it is located.
He is the customer who orders a Rusty on Rye, leading SpongeBob to perform a seance to meet Rusty Rickets and learn how to make the sandwich.
"High Sea Diving"
When SpongeBob finally makes it to the surface of the ocean, he finds Old Man Jenkins at the top with a bunch of junk. He tries to get SpongeBob to believe he is King Neptune and SpongeBob playfully tells him he is Poseidon.
Old Man Jenkins believed he was Poseidon at first, but soon realized it was really SpongeBob. He reveals that he accidentally ended up at the surface by getting pulled up by a bunch of balloons. When they end up getting back down to Bikini Bottom, the Bottomites end up believing that Old Man Jenkins is King Neptune, until the real King Neptune himself arrives and zaps Old Man Jenkins for impersonating him.
After SpongeBob's friends finish the lost Mermaid Man and Barnacle Boy episode recital, Old Man Jenkins chases them off for using his outdoor amphitheater. However, he just wanted to play along with them. SpongeBob gladly plays along with him and offers him the role as Mermaid Man. He happily accepts his role and flies off into the sky.
Mr. Krabs has SpongeBob and Patrick work at Old Man Jenkins' farm in order to pay off his grocery bill. They try to finish their assigned tasks and help him prepare for his upcoming barn party, but they end up failing everyone, as well as destroying his barn. After SpongeBob and Patrick do some barn raising, Old Man Jenkins ends up with a brand new barn for his barn party. The party starts out slow until a group of farmer aliens arrive and join the party.
Old Man Jenkins is pleased with SpongeBob and Patrick's help and offers them to come back and visit anytime.
Whenever people run away from Filthy Muck and Little Yuck because of their rancid smell, Old Man Jenkins is the only one to not run, since he has no sense of smell, thus causing him to be unaffected by their smell.
At the end of the episode, when Patrick reveals to have taken a bath after being kicked out of the Krusty Krab and is in his bath clothes, Old Man Jenkins gets frightened. SpongeBob questions him on this, as he tells him it is because Patrick's bath clothes fell off of him and his butt was being clearly seen by everyone. When everyone realizes this, they all laugh at Patrick's expense.
He comes to the Krusty Krab and disrupts business as normal, and Mr. Krabs tries to get him out. They call for their elders and later get kicked out of the Krusty Krab by them. They then have to go to the Chum Bucket, where Plankton tries to get them to leave but they hit him and tell to respect their elders.