Batman/The Flash by Joshua Williamson

The Button

After a mysterious smiley-face button turns up in the Batcave, a brutal attack from a time-bending foe forces two iconic heroes to join forces and chase answers across the Speed Force and into a fractured past. Their search leads through the Flashpoint timeline and a heartbreaking encounter with a long-lost father, revealing that their entire reality has been manipulated by an unseen, godlike power—and that the truth may exact a price neither of them is ready to pay.