Pearls are a Spoink's Best Friend - S7.E39

Continuing their journey to Fortree City, Ash and friends come across a Spoink, who is upset after losing the Pearl it keeps on the top of its head. After Spoink tries to put on a couple of round Pokémon on its head, the Pearl is found. However, during a Team Rocket encounter, they steal the Pearl. After coming across a fair, James is approached by the same Magikarp Salesman who has made his life miserable in the past and offers to trade a Feebas for the Pearl. After discovering Feebas evolves into Milotic, Team Rocket agrees. However, when they let their new Feebas out to have a swim in a nearby river, they discover the Feebas is just a Magikarp painted with a Feebas' colors, meaning they have been swindled yet again. After Ash and the others find out what happened, they have to get Spoink's Pearl back by finding the Magikarp Salesman before Team Rocket do.