TM24 Thunderbolt

You’ll obtain TM24 Thunderbolt by completing a request from Mauville City’s Gym Leader Wattson. He’ll ask you to check up on a computer in New Mauville, which is a power plant located underneath Mauville City.

But you can only do this sidequest once you’ve defeated/captured Kyogre or Groudon, depending on what version you’re playing.

Prerequisites

Before being able to do this sidequest, you should take care of these two prerequisites:

• Defeat/capture Kyogre or Groudon
• Have a Pokémon in your party that can use Surf

Location

To gain access to New Mauville and start the sidequest, head to Mauville City’s Pokémon Center. Wattson will be waiting for you in front of Square Tower.

And here’s the process to get your TM:

Step 1: Head south from the Pokémon Center and talk to Wattson.

Step 2: Go back inside the city’s interior using the north entrance.

Step 3: Walk west until you see an elevator on the north wall.

Step 4: Use the elevator to get to the second floor.

Step 5: Make your way toward Wattson’s apartment, located on the city’s south side.

Step 6: Follow the hallway heading south.

Step 7: Then turn right and continue moving east through the hallway.

Step 8: Enter the second apartment from the left, it should be on the northern wall (pictured below).

Step 9: Talk to Wattson, and he’ll grant you access to New Mauville.

Step 10: Then go to Route 110 using Mauville City’s south exit.

Step 11: Use Surf on the body of water near Cycling Road’s entrance.

Step 12: Surf east until you land outside New Mauville.

Step 13: Interact with the gate to enter the power plant.

Step 14: Go through the door that’s northwest of your current location.

Step 15: Interact with the computer in the middle of the room. Press “Yes” when prompted.

Step 16: Lastly, make your way back to Wattson’s apartment to receive your reward.