Giant Chasm - Crater Forest
From the Plasma Frigate south entrance, slide south, to the stairs. Move down them and slide south once more. Now slide east, north, east, north, east, south, west, south, east, north, then east onto the steps. Now move east, south, east, south, east onto the last set of stairs before you're off the ice.
Once you're off, follow the path north. Continue following it when it bends west. You'll reach some stairs. When you do, move up them and into the cave at the top.
Giant Chasm - Crater Forest Cave
Follow this cave, moving up and down the steps. Move through the doorway at the end, into the next area. Continue north and you'll have caught up with Ghetsis in a clearing within the cave. He'll summon Kyurem from the Frigate, which he has under his control.
Kyurem will go nuts, but N will come to the rescue along with his trusty Zekrom, if you're playing Black 2, or Reshiram, if you own White 2. What an unexpected turn of events! Eventually you'll switch to a cutscene of Kyurem and Zekrom/Reshiram facing off against each other.
Zekrom/Reshiram will be bound by Kyurem before fusing with it! Black Kyurem or White Kyurem will be formed! N and Ghetsis will continue talking. When they stop, guess whose job it is to stop Kyurem! Move up to it and interact with it to begin the battle. Thankfully, it's not all that difficult.
Note: Kyurem cannot be caught during this battle
Zekrom/Reshiram will reappear and Kyurem will have disappeared. Ghetsis is now raging and comes at you! Your Pokemon will automatically be healed before the battle. Game on.
Trainer Battle: Team Plasma Ghetsis
Cofagrigus is holding Leftovers
Hydreigon is holding a Life Orb
After beating Ghetsis, he can't suspend disbelief. He gets extremely agitated and angry and calls one of the Shadow Triad who then causes them both to vanish. N will talk with you and will eventually leave with his Zekrom/Reshiram. Hugh will follow in closely behind, but will have missed everything!
Answer his question and continue speaking with him. When you're done feel free to leave the cave.
Giant Chasm - Crater Forest
Outside, the Frigate will have left, leaving a mess of destroyed trees. That said, there's a few items you can get here that you couldn't get before. Move down the steps, head east, then south when you reach the corner. Continue down until you see the gap in the trees taking you west.
Head through them into the clearing where the Frigate was. Feel free to talk to the Old Team Plasma members. Once you're done, move into the Dark Grass a far way to their north-east. Move through to the other side. You'll notice an item to your south, between the dead trees.
There's a little gap between the eastern-most dead tree and the wall of trees east of it. Move through this gap and grab the item, which happens to be a mega awesome PP Up ! Backtrack back to the Old Team Plasma people and head into the Tall Grass just west of them.
Move through it and head north into the Dark Grass. Move over and grab TM03 - Psyshock then once again head back to Old Team Plasma. To their south-west is even more Dark Grass. Head south through this, then head east through the next patch.
Once you're past it, follow the thin path east through the trees. At the very end is a Star Piece . Grab it then skip merrily back to Old Team Plasma. Move north from Old Team Plasma. The first path of Tall Grass (not Dark Grass) you reach, move east through it.
Now move south-west down to the thin gap through the trees. Move through it then jump over the ledge taking you north. Follow the path around and move through the Tall Grass between the trees. From there, head north-east between the two patches of Dark Grass, then west into the north-west of the two patches.
You'll see a Carbos here. Grab it then head back through the Dark Grass. Now move east through the other patch of Dark Grass. Once you're past it, you'll find another item, a Sun Stone . Move back into the Dark Grass and head north until you're out of it.
There'll be a ledge to your right. Jump over it. Follow the path back around to the clearing and head back to Old Team Plasma one last time. Now head south a little, then west into and through the Tall Grass. Once you're past it, head south to the stairs. Move up them back into the cave.
Move up the steps, then south down to the southern wall. Follow the wall west. When you reach the corner turning north, continue to follow it. Continue until you reach the exit out of the cave to your left. Move through it and you'll be in Route 23 !