This issue is a variant of Batman (DC, 2016 series) #21 [Jason Fabok Nonlenticular Variant]. There exist further variants.
International edition cover depicts Batman and The Flash running. Due to trademark restrictions of the smiley face in the UK, the international edition could not feature a smiley face on the cover, so different art was used.
This story continues in The Flash #21 (2016 series).
Story credits include special thanks to Geoff Johns and Joshua Williamson.
Batman first discovered the smiley face button in the Batcave after Wally West reached out to him from the Speed Force in DC Universe: Rebirth #1 (DC, 2016 series). Also in that issue, we saw Saturn Girl in the 21st Century in the custody of the Gotham Police, who were holding her for mental evaluation because of her claims of being friends with Superman and claiming to know the future.
Batman and The Flash previously agreed to investigate the button and the disruption to their timeline that Wally revealed to them, as shown in Flash: Rebirth (DC, 2016 series) #1.
The letter to Bruce from his father Thomas that had been brought back from the Flashpoint universe by Barry Allen (as shown in the Flashpoint DC 2011 mini-series) was destroyed in this issue by Professor Zoom.