It's the episode where Misty and Brock leave. (although Brock comes back 4 episodes later, saves everyone, and makes them dinner)
Misty's cunt sisters phone her up when they get to Viridian City and tell her to come back and run the gym while they go on vacation. Nurse Joy tells her that her bike has been fixed. Misty is all sad about this, and Ash basically says "lol your bike is fixed you can leave now."
Brock just leaves because he "has some stuff to take care off," although in Pokemon Chronicles, it's revealed that he went home because his mom was trying to turn the gym into a water gym. He said "fuck that" and battled her sweet milf ass for it.