After you discover the secret of the Plasma Frigate's power, you must follow it to Giant Chasm.

Go to Giant Chasm

To reach Giant Chasm, go to Humilau City, then north to Route 22, and go to the northwest corner of the route to find the cave entrance to Giant Chasm.

Go through the Cave

Go south from the cave entrance until you encounter a couple of Team Plasma Grunts. Hugh will arrive (but only if you defeated Zinzolin on the Plasma Frigate earlier) and the Grunts will go south. Hugh will follow them, then the ex-Plasma spy will follow.

Go south. If you have a pokémon that knows Strength, use it on the boulder nearby to create a shortcut. If you walk over the boulder, go north, then Surf north on the water, you'll find TM13 Ice Beam.

Go back to the boulder that can be pushed with Strength and go west from there. If you don't have Strength, just go to the south path and walk west.

In the bottom-middle of the cave, there is an exit. Go outside using that south exit.

Once outside, go east and north to find a Yellow Shard. Use your Dowsing Machine to find a Moon Stone on the piece of bridge north of there. Now go south and go up the stairs to the right, then follow the path around and use your Dowsing Machine to find a Max Potion hidden in the grass.

If you continue south, you end up on Route 13. If you go south and west, you find a boulder that can be pushed with Strength. Push it into the hole to create a shortcut to Lacunosa Town to the west. Go east and south from the Strength boulder to go down some stairs leading to a Hyper Potion on the beach. Use your Dowsing Machine to find a Heart Scale nearby. Near these stairs, there is a hidden grotto in the trees. Feel free to rest up in Lacunosa Town, then go back north to the Giant Chasm cave.

From the southern entrance of Giant Chasm, use your Dowsing Machine near there to find a Star Piece. You can go west from here and push a Strength boulder to get a Max Repel. If you keep going west, there are a couple of Team Plasma Grunts, but they won't move.

Use your Dowsing Machine and go north from the Grunts to find a hidden Ultra Ball. Push the nearby boulder with Strength to get a Star Piece. To the east, push another boulder to get a hidden Ice Gem.

Continue following the path to end up where you started. Jump down the ledge and go to the bottom-middle of the cave. Use the stairs down to the deep hole and use the cave exit there.

There are a bunch of people outside. Go up to Hugh and he'll break up the argument, and you'll have to fight a Team Plasma Grunt who sends out a level 47 Weezing and a level 47 Muk.

After you win, Rood gives you three Max Revives. Rood tells Hugh who probably has his sister's pokémon. Hugh runs off to the northeast. Go to the southwest to get the Razor Claw sitting there. Then go to the northwest to get a Moon Stone.

Now go northeast to where Hugh went. Go up the stairs and you'll slide north for a while. Go to the right and up the stairs, then east, north, west, and south. Then go west, south a few times, and east into the Plasma Frigate.