According to KingDrapion's development thread:
The story of Pokémon Hoenn White 2 takes place two years after the events of Pokémon Black and White. The game begins with the player character receiving a Pokémon from Professor Juniper, a renowned Pokémon researcher. The player soon learns that a group of Pokémon, known as the "DNA Splicers," has stolen the DNA of Pokémon, causing chaos in the Hoenn region. pokemon hoenn white 2 documentation