Operation Z.E.R.O.: Out Mandy'D is a Billy and Mandy online game. It is based on "The Grim Adventures of the Kids Next Door".
The game is essentially a remake of Operation: Z.E.R.O.: Out-Numbuh'd!" which was made to promote the KND movie "Operation: Z.E.R.O." The only difference in the game is that you fight as Mandy rather than as Numbuh 1 and that the intro and ending sequences are different, and that all references to Numbuh 1 are replaced with "Mandy."