A doppelganger is a negative counterpart of a Mario character and the doppelganger's names are always the backwards names of the character that they are based off. The team of doppelgangers is called Team Doppel.
Mario did Princess Peach's work and he was tired of it. Mario couldn't take it anymore and created a clone to do his work. Mario told Peach that he has a clone to do the work. Peach was pleased with the news and ordered the clone to come to her. The clone came and obeyed Peach's commands. But at night when Peach was sleeping, The clone awoke from his bed and named himself Oiram. Oiram then went back to bed. A koopa snuck in the castle and changed Oiram to obey Bowser. Oiram didn't come to Bowser. The koopa made Oiram a monster and a doppelganger of Mario. Oiram then brainwashed all the main characters of Earth to insert in Oiram's cloning machine. Then Oiram un-brainwashed them all and began his army of doppelgangers. It's up to the heroes to stop Oiram.