King Neptune's crown, unlike his one from the regular episodes, is red and golden in color with a fishbowl-like shape.
Role in movie
King Neptune greatly cares for his crown mainly because it covers his bald even (despite constantly saying it's "thinning"). He even has a Royal Crown Polisher to polish the crown and tries to have him executed just for touching it (as that's his job to polish the crown). Fortunately, Mindy lets the polisher go much to King Neptune's fury.
Later, Plankton steals King Neptune's crown and blames it on Mr. Krabs as part of his Plan Z scheme. King Neptune then heads for the Krusty Krab to confront Mr. Krabs about his crown. However, when Mr. Krabs says he doesn't have the crown, King Neptune shows a picture of Mr. Krabs saying he has stolen it. It doesn't help Krabs much when someone on the phone (Plankton) calls thanking Mr. Krabs for selling him the crown which is now in Shell City. King Neptune is then furious to hear that his crown is at Shell City and prepares to execute Mr. Krabs. However, SpongeBob steps in and agrees to get the crown back for him. King Neptune is later convinced and allows SpongeBob six days to get back the crown (it was originally ten but Patrick kept reducing the numbers). SpongeBob and Patrick then set out to Shell City to get the crown.
SpongeBob and Patrick go through many trials throughout their journey. They are eventually captured by The Cyclops and are taken to Shell City. Knowing where they're at, SpongeBob looks over to see King Neptune's crown, which is being sold for $7.00. SpongeBob and Patrick then dry up on the table, accepting their fate. Fortunately, their tears allow the sprinkler system inside Shell City to turn on and bring them back to life. As the other revived creatures attack the Cyclops, SpongeBob and Patrick take the crown and leave Shell City.
On dry land, SpongeBob and Patrick attempt to head straight back to the Krusty Krab, but Patrick opens the Bag of Winds too soon and it flies away without them. Fortunately for them, David Hasselhoff appears and agrees to give them a lift. As SpongeBob and Patrick ride on Hasselhoff, they are faced with one last roadblock: Dennis. SpongeBob and Patrick manage to evade Dennis to the point he is knocked off by a passing sailboat. As King Neptune prepares to kill Mr. Krabs, David Hasselhoff fires SpongeBob, Patrick, and the crown down to the Krusty Krab, where the crown deflects King Neptune's blast onto Hasselhoff.
King Neptune is happy to get his crown back but Plankton soon controls him with one of his Chum Bucket Bucket Helmets. SpongeBob comments that Plankton "cheated," but Plankton says he won because he's a genius and SpongeBob is just a kid. SpongeBob then does a monologue before bursting out into song ("Goofy Goober Rock") and destroys all of the bucket helmets. Mindy then gives back the crown to King Neptune and Plankton is soon arrested.
Role in series
Outside of the movie, King Neptune's crown has also appeared once in the series. In the episode "Lost and Found," the crown makes a cameo among the many items placed on the Lost and Found's shelves.