The Simpsons (1989): Season 18, Episode 8

18 - 8: The Haw-Hawed Couple

After Bart convinces all his friends not to go to Nelson's birthday party, Marge forces him to attend. After the party, Bart becomes Nelson's new best friend and under Nelson's protection no one dares to mess with Bart. There's only drawback to his newfound friend/bodyguard ; he can no longer pal around with Milhouse.