Heroes from the Marvel and Capcom universes band together to prevent Ultron Sigma's reign of terror over their worlds.

Believing all biological life must be purged, Ultron and Sigma forge an alliance. Using the Reality and Space Infinity Stones, they fuse themselves into a single being named "Ultron Sigma", and merge the worlds of Marvel and Capcom into a single dimension, which they seek to rule by unleashing a cybernetic virus that puts others under their control. As a result, the heroes of both worlds must band together to retrieve the remaining four Infinity Stones with the now-captured Thanos' help, and defeat both Ultron Sigma's forces and a deadly group of villains led by Jedah Dohma, who wish to unleash terror on the merged world using the power of the Symbiotes.