The Powerpuff Girls season 1 episode 22
Episode title: Odd Bubbles Out
Donny the Unicorn and Big Joey participate in the Student/Unicom Exchange Program. Bubbles has many planned activities but Donny is doing them with Chelsea. At first Bubbles is upset but decides to meet Her since Donny likes Chelsea so much. Buttercup thinks Chelsea is a Bubbles 2.0. Donny and Bubbles argue later that night and Donny decides to stay at Chelseas place. Trying to comfort Bubbles; Blossom and Buttercup realize Donny is likely in trouble and set out to try and save him
