A Scare to Remember : Season 7 Episode 49

Yet another Team Rocket plot fails to go off quite as planned, but this time the error works to their advantage. When their latest overwrought invention explodes, the ensuing shockwave not only sends the team of villains flying off into the horizon, but also wipes Pikachu's memory clean. As Meowth is the first one to stumble onto Pikachu and learn of its plight, he quickly turns the situation to his advantage, informing Pikachu that it is a long-standing, loyal member of Team Rocket and that Ash is his sworn enemy.