Spike sneaks into Zecorah's place and 'borrows' a special flower. Smelling it will make you fall in love 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 castle to see what the ruckus is about. When she too smells the flower and falls in love with Spike all hell breaks lose.
Spike is forced to ask Zecorah for help. She creates a pink cloud of fog, that cures everypony. Luckily for Spike nopony remembers a thing. In the end he writes a letter to Celestia, about how love is something that cannot be forced.