Tekken Tag Tournament, being a non-canon game, features no storyline. It is more of a compilation of the Tekken series giving fans the opportunity to play as almost every character in the series up to that point.
Much debate has come from fans regarding the game's boss character Unknown. Most speculate that Tekken Tag Tournament features all the fighters exsisting in a dream world, all coming to together (and subsequently) teaming up to fight a common evil.

Some consensus regarding these speculations are evident by Unknown's CG ending, where she (apparently) escapes from the Wolf demon sprit possessing her body, as well as Ogre's ending depicting him (it) about to engage in a fight with Unknown. However, only further games may tell if these speculations (and Unknown's history) are notable to any canon.

With 2002's WWE Raw PC being the only viable Wrestling PC game around, the game filled a void for many players who could now have a real wrestling PC game. but that void lasted for six years, although there was many mods coming out, there was not yet a FULL conversion of the entire game, (WWE Raw: Ultimate Edition was aiming for this but was never released). Later RMU(Now TWR) released Raw Legend edition and Raw Total Edition which were pretty good.