ITT: We Are All Pokémon Trainers, or WAAPT for short, is a Pokémon Play-by-Post Game created by gamerex27 on February 7, 2011 where all of the participants are Pokémon Trainers. The plot starts off with a Talking Heads bit about all the people randomly showing up in an airport headed to Sinnoh. Then Missingno shows up, turns Canalave City into a mountain of eggs, and Mewtwo shows up to tell the group that they need the help of Legendary Pokemon to defeat the Anomaly. Things just continue to get more insane from there.
In a long-running epic Fanverse which has so far included a pair of in-universe Mary Sues, a group of Pokemon researchers, a former NPC, a Mystery Man by the name of Pentigan, ex-Pokemon Rangers, a Dark-Type Gym Leader in training, immortals, people with multiple personalities, a former coordinator, Pokémon that turn into humans, humans that turn into Pokémon, superheroes, former evil team members (or their children), more ex-champions and monotype trainers than you can shake a stick at, as well as the children of another ex-champion, basically the whole of the known Pokéverse (and some of the unknown portions), virtually every Legendary Pokémon in existence, and alternate universes, arcs are common and epicness ensues.
Due to the RP evolving from a forum game with no real canon, the events happening in the earliest arcs are slightly subject to Broad Strokes Canon and Early Installment Weirdness, and may not accurately represent the current state of the RP.
It also has a wiki.
Also, please note that spoilers are unmarked for any events or reveals earlier than the past two arcs.