The reason that they are enemies is that when they were children they fought over a recipe that they made and Mr. Krabs got most of it and Plankton only got a piece that read "chum". Mr. Krabs accidently hit a wall and ingredients that were on a shelf fell into patty batter that they made and the Krabby Patty was made. The next day, Mr. Krabs and Plankton went to school with patty batter. Kids tried both Mr. Krabs and Plankton's batter. The children preferred Mr. Krabs' patty. That is when Plankton vowed to steal the formula to run Mr. Krabs out of business.