April's Fool (S03E11)

Emperor Aleister of Malicuria comes to New York City, bringing along a shard of Lydium 90 to the USA. In order to get the mineral to fuel the Technodrome, Shredder invades the costume party thrown by the Malicurian US Embassy to kidnap Aleister's daughter, princess Mallory, but ends up capturing April in a fancy dress who looks just like Mallory. (iTunes)