Team Rainbow Rocket arrives to the region where you are (Guzma and Piers come with them also, dont ask why), and they take over your region.

But their internal rivalities made them separate and form their original 8 evil teams. The region now is fragmented in 8 fractions and each one of those is under the absolute control of one team, fight them is useless.

As a trainer you have 2 options, join one of them or get robbed of all your pokemon. What Team are you joining?