SpongeBob is jellyfishing and tells a jellyfish to stay off the road because it's dangerous. Some men on motorcycles drive past him, and SpongeBob holds on to one of the men's jackets, ripping a 'M' off of the jacket. He looks at the jacket when he has to let go of it. He presumes it says "The ild Ones," but he soon notices the "W" in his hand and figures out that they are called "The Wild Ones." After someone tells him about "The Wild Ones," SpongeBob runs to tell Mr. Krabs (actually, SpongeBob "takes the shortcut, which is a piggyback ride on a bubble).
He does not listen, so he tells Patrick. Mr. Krabs puts up a banner saying 'WELCOME WILD ONES!' on the Krusty Krab and raises his prices in order to get them to eat there after hearing SpongeBob described them as "ravenous."
After Patrick says that if they dressed up like bikers, they would be the big scary bikers, SpongeBob and Patrick decide to dress in biker attire and ride into town on a paddle-unicycle and a paddle-tricycle, respectively. After the bikers arrive at The Krusty Krab, it turns out that they are old folks.
The actual name of the biker gang is 'The Mild Ones'. SpongeBob had the 'M' upside-down. The biker that had the "M" ripped off of his jacket gets his "M" back from SpongeBob, and The Mild Ones ride away. Shortly after this, it is revealed that Squidward decided to join with them.
SpongeBob and Patrick's attire were inspired by the 1953 film The Wild One.
Coincidentally, the episode title is similar from the Squirrel Boy episode "Born to be Mild", but in the SpongeBob episode a group of motorcycle gang was named "The Mild Ones."
The title was the wordplay on the Warner Bros. Family Entertainment film Born to be Wild which came out on March 31, 1995, exactly 12 years before this episode premiered.
But, the film's plot was different than this episode's plot and Easy Rider,Born to be Wild's plot.
When SpongeBob barricades the Krusty Krab's front door with the cash register station, Squidward is sitting in the boat. However, a moment later SpongeBob finds Squidward at home when he rushes to Squidward's house to warn him of the biker gang. It is possible Squidward went home in between these scenes.
In addition, Squidward would've already heard about the biker gang because he was in the boat. So, SpongeBob didn't have to go to Squidward's house afterwards.
When the Mild Ones are driving around Squidward, when you watch that in slow motion, you can see that the men's jackets say "Mile Ones" instead of "Mild Ones."