Pokémon Black 2 and White 2 picks up the story of Unova two years later, as new trainers begin their Pokémon journey. Unfortunately for them, Team Plasma has returned, this time broken into two factions vying for power and control of the Unova region. With the evil leader Ghestis attempting something even more sinister this time around; harnessing the power of the legendary Kyruem in its true form!
Featuring brand new post-game content that rivals the best of the Pokémon series, the new Pokéstar Studios, and more online features than ever before, Pokémon Black 2 and White 2 are considered some of the best games in the entire franchise, and the last in the series to use in-game sprites before the transition to 3-D models.
From the game's Wikipedia page:
"Pokémon Black Version 2 and Pokémon White Version 2 are role-playing video games developed by Game Freak, published by The Pokémon Company and Nintendo for the Nintendo DS. Part of the fifth generation of the Pokémon video game series, the games are direct sequels to Pokémon Black and Pokémon White, being the first sequels in the series. They were first released in Japan in June 2012, with a worldwide release following in October 2012. The games were first revealed on the February 26, 2012, episode of the Japanese television program Pokémon Smash!, followed by an international confirmation on the Pokémon official website. The games feature the legendary Pokémon identified by Junichi Masuda as Black Kyurem and White Kyurem.
A “Pokémon Grey” game was not released due to the title clashing with the theme of opposites found in Black and White.
Pokémon Black 2 and White 2 are set two years after the events of Black and White, and much of the beginning events take place in new locations on the western side of the Unova region. These new locations also feature several Pokémon that were previously unavailable in Black and White, featuring a total of 300 unique creatures available from the start of the game."