Cheese Sandwich is at an awesome party he threw when suddenly a mysterious mare comes in singing a song. The ponies love her so much they put her on stage, only to discover she's a siren! The siren steals everypony's spirits, their party spirits. Cheese Sandwich won't stand for this! So he goes on an incredible journey, accompanied by some faithful party ponies, to defeat the siren and get everypony's party spirits back!