After you defeat Drayden in the Opelucid Gym, there is a disturbance in Opelucid City.
After Defeating Drayden
Outside, Drayden will come out of the gym and ask you to follow him. Go to Drayden's house, to the left of the pokémon center. You can heal up in the pokémon center before you visit Drayden.
In Drayden's house, Drayden will tell you about the legendary Dragons. Zekrom pursued the ideal of bringing a new world of hope. Reshiram pursued the ideal of bringing a new world of goodness. The two dragons used to be a single dragon. They helped two heroes create Unova. The older brother sought truth while the younger sought ideals, but they couldn't agree, so they ruined the land as they fought. The dragons split apart to represent ideals and truth, but those two ideas need not be in opposition.
A third dragon, Kyurem, came into existence at that time. The proof of this could come from Drayden's treasure, passed down for generations: the DNA Splicers, which Professor Juniper proved to have come from the era of Dragonspiral Tower. The DNA Splicers are well-protected, but Drayden is unsure what power they hold.
Drayden isn't sure that a third legendary Dragon could exist or if it would even be powerful, because if it is part of the single legendary Dragon, it might just be a shell or husk.
Suddenly there is an explosion, and the Plasma Frigate flies overhead. It shoots giant ice crystals all over the city.
Drayden's Haxorus tries to break the ice with Dragon Tail but it has no effect. Zinzolin and some grunts arrive and explain that their special technology created the ice, so it's unbreakable. Zinzolin demands that Drayden hand over the DNA Splicers. Drayden refuses, and Zinzolin can't shoot more ice until the weapon recharges, so Zinzolin and the grunts leave to go look for the DNA Splicers.
Drayden asks you to go fight the Team Plasma Grunts.
Go to the right of the giant crystal and go south. You'll slide on the ice and end up near the pokémon center. Go into the pokémon center to heal up and switch pokémon if you want.
Back outside, fight the Team Plasma grunt to the east of the pokémon center. He has a level 44 Watchog and a level 44 Muk.
Now slide north on the ice, as far north as you can go. Slide west to the Team Plasma Grunt in front of the door. She has a level 44 Golbat and a level 44 Garbodor.
Now go west, then south. Go slightly to the left, then up to reach a Team Plasma Grunt who has a level 44 Seviper and a level 44 Weezing. This grunt mentions that they're searching the pokémon gym, so go north to the gym.
In front of Opelucid Gym, you will find Zinzolin. Talk to him. He has two level 46 Cryogonals and a level 48 Weavile. Against the Cryogonals, which are vulnerable to fire, steel, rock, and fight, I recommend using Litwick/Lampent/Chandelure. Against the Weavile, which is vulnerable to fight, steel, fire, rock, and bug, I recommend Magnemite/Magneton/Magnezone, Cobalion, or Pawniard/Bisharp.
After you defeat Zinzolin, he will leave. Drayden heals up your pokémon, then gives you the DNA Splicers. But then the Shadow Triad suddenly appears and steals the DNA Splicers, then runs off. Go south, then east to find a Shadow Triad member. He has two level 46 Pawniards and a level 48 Absol. Against the Pawniards, which are very weak to fighting moves and are somewhat weak to ground and fire moves, I recommend using Cobalion or Lucario. Against the Absol, which is weak to Fighting and Bug moves, I recommend using Pawniard/Bisharp, Scrafty/Scraggy, or Genesect.
After the battle, the Shadow disappears and Drayden runs over, apologizing for letting everyone down. You then get a call on the Xtransceiver from Hugh and Cheren. After you hang up, Cheren arrives. He says that he thinks Team Plasma is near Humilau City. Drayden says it's north of Undella Town.