SpongeBob and Patrick are in Jellyfish Fields when they hear a strange noise. Soon, a stampede of large whelks make their way through Jellyfish Fields, destroying anything in their path, including SpongeBob and Patrick. The whelks covered them in purple goo, and proceeded. As the whelks made their way to Bikini Bottom, SpongeBob and Patrick decide to warn the Bikini Bottomites about the whelks, but too late. Back in Bikini Bottom, the whelks were chasing and terrorizing everyone. They would blow purple goo bubbles around their victims and swallow the bubbles with their victims inside. SpongeBob and Patrick were asking who are those monsters. But Patrick was never scared of them.
Johnny Elaine explains them on TV, suddenly he signs off the news when he also has been eaten by a sea whelk. When SpongeBob and Patrick got to the Krusty Krab, they warn Mr. Krabs about the coming danger. Soon after, the whelks make their way to the Krusty Krab, but Mr. Krabs thinks they are just plain old customers.
A larger mother whelk is then revealed, assisting her "babies." When he offered them all a Krabby Patty, the mother swallowed him, along with Squidward and the entire Krusty Krab. SpongeBob and Patrick then ran to Sandy's treedome, where after being tested that they were SpongeBob and Patrick and not the whelks, they went inside. Sandy put metal armor around the Treedome for protection against the whelks and told SpongeBob and Patrick her plan to defeat the whelks.
Suddenly, the whelks crack and destroy the Treedome and SpongeBob, Sandy, and Patrick are swallowed by the mother whelk. Inside the whelk, Sandy discovered that the whelks were sick, or "had the sniffles." SpongeBob then simply helped the whelk get better again by doing what he did to cure his Suds, revealing the whelk's true form, a tiny, helpless whelk. SpongeBob does this to all the whelks and restored the whelks' happy family. All of the "eaten" victims were rescued, and the episode ends with Squidward complaining about all that just because the whelks "had the sniffles," then getting doused with purple whelk goo.