It's the Plasma Frigate, just like the last time you've seen it, except none of the Team Plasma Grunts are feeling grunty enough to fight you. If you missed any items before, you can go back and pick them up now.
The main place you want to go, though, is inside the door on the southern part of the frigate — not down the stairs there, if they're open for you, but just in the door. Take the teleporter there and then head north. You'll find a little collection of grunts. Back there, head east and take the teleporter to find Colress again! He's stationed up where you fought him the last time. Talk to him again and he'll challenge you to a battle, thankfully playing his awesome music instead of the ordinary Trainer battle music.
||PKMN Trainer Colress||$14800|
His Magneton likes to open up the fight by paralyzing you with Thunder Wave, then get out of the way with Volt Switch. Keep in mind that he also has Full Restores that he likes to use, too, so you may have to whittle his Pokemon down several times.
His Wash Rotom can be a sneaky little thing with some curious typing. It has Hydro Pump to do some massive damage. It packs a Life Orb to increase the power of its moves, too.
The Magnezone has a lot of Special Attack, plus packs Thunderbolt, Flash Cannon, Hyper Beam, and even Thunder Wave for some crippling paralysis. Sturdy ensures it will survive any attack if it is at full HP, too, so watch out for that.
Beheeyem loves to open up with Calm Mind to raise its Special Attack and Special Defense, while it also packs the Leftovers to heal its HP.
After beating Colress, he will give you... a Master Ball! Yes, that's right, one more of the most powerful variety of Poke Ball in the entire game. As you can see, it's definitely worth taking him on.
Well, that's all there is to do on the frigate, so head on out of the Plasma Frigate and then head west of the P2 Lab to get back onto Route 17 to wrap everything up there.