The western section of Route 210 is foggy, which reduces the accuracy of all Pokémon in battle. You can use HM05 (Defog) to clear the fog, which you'll receive later. For now, navigate carefully and battle the trainers. Continue west to reach Celestic Town. In Celestic Town, you'll encounter Cynthia's grandmother and a Team Galactic Grunt. After defeating the Grunt, Cynthia's grandmother will give you HM03 (Surf) if you haven't received it already, and explain the history of the town. You'll also receive the Old Charm. Your next destination is Canalave City. Head west from Celestic Town onto Route 211.
