Spike sneaks into Zecorah's place and 'borrows' a special flower. Smelling it will make you fall in amor with the first person you see. Obviously he tries to get Rarity to smell it, accidentally infecting everyone in ponyville in the process. Eventually everypony's chasing Spike in a lovestruck stampede, causing Celestia to come down from her castelo to see what the ruckus is about. When she too smells the flor and falls in amor with Spike all hell breaks lose.
Spike is forced to ask Zecorah for help. She creates a rosa, -de-rosa nuvem of fog, that cures everypony. Luckily for Spike nopony remembers a thing. In the end he writes a letter to Celestia, about how amor is something that cannot be forced.