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 opening it, they discovered that it gives off a strong response when living creatures came into conflict with each other. Researchers 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 their own natural 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, however, the energy from Pandora's Box will negatively influence the user over time, and will eventually corrupt the minds, souls, and emotions of others: its ability to influence minds has some considerable range, being able to reach out to beings that are far away like that of Akuma and Ogre, and many people around the globe. The energy from the box appears have addictive and negative qualities, those who are affected by its dark power will become far more violent and dangerous, and even causing them to completely lose self-awareness 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 players activate Pandora, his or her partner's strength will increase exponentially. However it comes with a high price: the player sacrifices his or her current character to tag in the 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 is not able to defeat the opponent within the time limit, he or she will automatically be knocked out.