The story of Street Fighter X Tekken begins with a mysterious meteor that crash lands in the Antarctic. An unnatural object in the shape of a box was discovered among the fragments of the meteorite. Researchers from all over the world have studied the object, and despite not finding any conventional way of openning it, they discovered that it gives off a strong response when living creatures came into conflict with eachother. Researcher's have nicknamed the object Pandora.
The power of Pandora, in addition to granting the personal wishes and hidden desires of others (as seen in the Street Fighter X Tekken Vita episodes), can cause the power of a person's natural strength to grow to levels beyond human limits (as seen with Dudley, who was able to punch a Pandora infused mammoth from a faraway place), and can also awaken and merge (and potentially influence) with other inner powers, in turn, the energy will negatively influence the user, and will potentially corrupt the minds, souls, and emotions of others: its ability to influence minds has some considerable range, being able to reach beings far-away like that of Akuma and Ogre from a faraway place, and many people around the globe. The energy from the box appears have addictive and negative qualities, making those affected by its dark power far more violent and dangerous, and even causing them to completely loose self-awarenss of themselves and absolute control over their actions, when under the influence of Pandora, the individual will have markings on their body, their skin will glow a blackish color, their eyes will glow red, and their voice will grow somewhat darker (though more of an echo).
When the player activate Pandora, their partner's strength will increase exponentially. However it comes with a high price, the player sacrifice their current character to tag in their partner into Pandora Mode. Pandora is a system with a built-in time limit. In Pandora mode, the Cross Gauge will be filled to maximum capacity, and the player will be able to use EX and Super Arts infinitely. However, if the player are not able to defeat their opponent within the time limit, they will automatically be knocked out.