The episode starts at a normal day at the Krusty Krab. Mr. Krabs is doing paperwork and is confused as to why his profits are down. He goes out into the restaurant to see Squidward making a sale, minus the price of a drink. Mr. Krabs sees everyone is drinking a thing in a bottle, including SpongeBob. He tells Mr. Krabs about the drink called the "Kelpshake." Mr. Krabs worries that people aren't buying drinks with their food, so he bans outside drinks in his restaurant, forcing the customers to leave.
He goes to see what Plankton has done this time. He doesn't know Plankton isn't behind this. Plankton tells Mr. Krabs he has no idea what is going on because he was relaxing in a beach chair. Then the two try to steal a Kelpshake and discover what it is made of. They go to a Kelpshake stand and try to get Plankton to find out, and all attempts fail. That night they both try to enter but an alarm wakes up a guard worm who attacks them. The next day Mr. Krabs reluctantly buys a Kelpshake and goes to the Chum Bucket to analyze what is made of.
Karen starts analyzing, but Plankton gets impatient and interrupts the progress of the analyzer. They find out that the drink has two ingredients, the primary one being kelp juice. The two drink the shake and find it to be delicious, but ignore Karen, who tries warning them about the other ingredient in the drink. After going out to buy some more kelp shakes, SpongeBob runs out of the Krusty Krab, wailing and crying. He is covered in green fuzz all over, and his clothes are gone. He also tells them that they're closing down the Kelp-Shake Restaurants and he runs away again. Then they go out and see that all Kelpshake restaurants are closed because their shakes contained a toxic and dangerous ingredient. Two decontaminants revealed that it will take decades to get rid of the unhealthy toxic waste and they're sorry for whoever drank the stuff. Then Mr. Krabs and Plankton grow the fuzz all over their body. So Plankton decides to go back to stealing The Krabby Patty formula and Mr. Krabs gives him a Krabby Patty and then chases him just for the fun of it.
This episode aired in Germany on November 18, 2006, before it aired in the USA.
This episode was originally going to be called "Kelpshakes."
The title is a mixture of "Best Friends" and "Enemies," giving "Frenemies."
The expression "frenemies" is inspired by Elizabeth Everitt's term of her crush Bailey Valint and her acquaintance Shane Griffin. She is a friend of the creator's daughter.
The customer who just ordered a Krabby Patty only paid a dollar for it. This episode may have taken place on a Wednesday because according to SpongeBob back in the episode "Imitation Krabs," a single Krabby Patty only costs a dollar on Wednesdays.
Kelp Shakes have actually appeared in the series before, the Krusty Krab has had them on stock according to the menu in "Mermaid Man and Barnacle Boy V." They are presumably normal shakes but with the same name.
The Kelp Shake buildings divide like bacteria and seem contagious.
It is not known why SpongeBob was hanging around the Kelpshake Store at night.
The tannoy which warns Mr. Krabs and Plankton says You have three seconds before spontaneous combustion! and then pulls out a laser, but in real-life, spontaneous combustion means something that suddenly bursts into flames.
The last time the Chum Bucket was shown, it was destroyed by the Kelp Shake building, but it was shown again later when Mr. Krabs and Plankton tried to analyze the ingredients.
On the August 15, 2010 airing of this episode on Nickelodeon UK, at the end, Mr. Krabs says: "Hey, hahahahahahahaha!" while in other countries like the US, he says "Hey, get back here you little booger, Hahahahahahahaha!" to Plankton. This is because the word 'booger" sounds similar to "bugger" which is a word deemed profane in the UK, while in other English-speaking countries it is a nickname for a mischievous child.
This is the first time Plankton and Mr. Krabs unite due to a new restaurant having many costumers. The second is in "The Other Patty."
This is the first episode where Plankton and Krabs assist each other. The next two are "The Other Patty," and "Eek, an Urchin!"
This is the first time Mr. Krabs has interacted with Karen.
This episode is the second-to-last Season 4 episode to air, in airing order (not production order), first-to-last being "Squid Wood."
Kelpshake stands are a parody of Starbucks coffee chains, which have a reputation of suddenly popping up in large numbers all over the place, but don't instantly multiply like Kelpshake restaurants.
Plankton's abnormal hair growth makes Plankton have a head-hairstyle similar to Albert Einstein. Since Plankton has an interest in science, this hairstyle that he has developed is rather suitable.
When the guy who said "Wow, what a steal" ordered food, he just walked away without taking any of the food he ordered.
When Squidward says "We haven't sold a single soda for weeks!," the closed-captions say "We haven't sold a single Solar in weeks!"
When SpongeBob sucked the Kelpshake for the first time, the straw was outside his mouth.
At the part where Mr. Krabs sneezes, you can see an employee sign says "Employes Only." It is missing the third "e." Also, the sign was purple. But when Plankton landed, the sign is yellow and "employee" is spelled corrected.
After Mr. Krabs says Hey, get back here you little booger. Plankton disappears before the whole thing cut to blank.
There were three Kelp Shakes next to the Krusty Krab, two on the left and one on the right, but when SpongeBob was talking to Mr. Krabs and Plankton inside the Krusty Krab, two of Kelp Shakes on the left outside were gone.
Same as "New Leaf" and the second half of "Friend or Foe," Plankton's mouth is purple.