Pokémon Season 1 Episode 21

As the group continues on their way, they notice a flock of Butterfree out over the ocean. Brock informs Ash that it is the mating season for all of the Butterfree. Ash decides to release his Butterfree so that it can mate. However, Team Rocket has plans to steal them all. Ash’s Butterfree bravely frees all the Butterfree and in the end Ash releases it.