Plankton's chum is the signature ingredient in all of Plankton's menu items. As one of several major factors that causes the Chum Bucket to attract no customers, the taste is highly considered foul in contrast to Mr. Krabs' Krabby Patty. Although mentioned in previous episodes, chum first physically appears in the episode "Friend or Foe."
Role in series
During Plankton's childhood, he and his former best friend, Mr. Krabs, saw how their classmates' favorite burger joint gained "li-spect" and wealth. Pursuing their own yet combined goals, they attempted to open their own restaurant with their own recipe and burger, which turns out to take no attention. Their burger poisoned Old Man Jenkins, resulting in an argument that sparked their hostile relationship.
Mr. Krabs made his own burger, the Krabby Patty, and Plankton used chum for his burger. Plankton's creation proved foul even to those who can eat anything. On the other hand, Mr. Krabs' tasted quite good.
Plankton continues to use chum in most of his products, despite its foulness and overall toxicity (many have vomited after eating it). In later seasons, his chum was still inedible but another quality in his restaurant, such as the Chum Caverns or a popular slogan, would draw in customers.
In "Krabby Patty Creature Feature", SpongeBob learns that the cure to the Secret Patty's side-effects is actually chum. He then uses it to return everyone back to normal after they mutated into Krabby Patty zombie monsters, which helped him save the day.
The Chum Bucket contains a variety of products that are mostly made out of chum:
As revealed in "Jailbreak!," chum can actually be used to help pull off crimes and as a result, the criminal community holds Plankton in high regards.
Out of all the chum-related food, the Chummy Patties and Chum Fricassee are the only ones that Plankton has sold that actually taste good.
In "Free Samples," it is revealed that Patrick Star is the only one who can eat chum compared to anyone else who eats it. Also, in "Chum Caverns," Patrick ate a lot of Plankton's chum, which means he is the only one who loves eating it, though he doesn't know the food item is sold at the Chum Bucket.
Despite the fact that the ingredients for chum (as explained by Plankton in the episode "Spongicus") are only just that, chum, many other things have been seen in it such as an eyeball, as seen in the episode "Drive Thru."