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One Piece is a Japanese anime television series produced by Toei Animation that premiered in October 1999. It is based on Eiichiro Oda's manga series of the same name. The story follows the adventures of Monkey D. Luffy, a boy whose body gained the properties of rubber after unintentionally eating a Devil Fruit. With his crew, named the Straw Hat Pirates, Luffy explores the Grand Line in search of the world's ultimate treasure known as the "One Piece" in order to become the next Pirate King.

Since its premiere in Japan, more than 1,000 episodes have been aired, and later exported to various countries around the world.

Source: Wikipedia

Episode 870 "A Fist Of Divine Speed — Another Gear 4 Application Activated!"

When Monkey D. Luffy travels to Whole Cake Island, he's fresh off the heels of victories against Hordy Jones, Casesar Clown, and Donquixote Doflamingo. Because of this recent success, Luffy sets his sights on his biggest target yet: an Emperor of the Sea. However, when the plan to take down Charlotte Linlin goes awry, the Straw Hat Captain is forced to take on her strongest subordinate, Charlotte Katakuri, in a one-on-one fight.

Katakuri isn't as strong as his mother, but he's more than capable of pushing Luffy to his limit. The pair's battle easily ranks as one of the best in One Piece, and thanks to the dazzling display put on by Katakuri and Luffy's respective Devil Fruits, the final moments of the fight are a blast to watch every time. Episode 870 ends as each pirate lands one final punch, but even more importantly, it also provides a memorable debut for Luffy's Snakeman transformation.

Source CBR