4x12 - Justice League

With most of the League unavailable, the League's original seven battle against the fused Luthor and Brainiac. It is then up to the Flash to stand up against the Luthor-Brainiac team, and he is prepared to pay the final price. In desperation, Flash pushes his powers to their limit, running as fast as he can around the world numerous times and tapping into the Speed Force in order to separate and defeat the two villains. As a result, Flash is very nearly pulled into the Speed Force, but is rescued by the teamwork of his teammates. The incident leaves Flash with the disturbing possibility that traveling that fast again might mean he would never come back.