This issue is a variant of The Flash (DC, 2016 series) #21 [Jason Fabok Lenticular Cover]. There exist further variants.
Features same art as lenticular cover
Story continued from Batman (DC, 2016 series) #21 and continues in Batman (DC, 2016 series) #22.
Story credits include special thanks to Geoff Johns and Tom King.
The smiley face button first appeared in the Batcave when the original Wally West returned in DC Universe: Rebirth #1 (DC, 2016 series), and that is when Batman and the Flash first learned of an outside influence manipulating the timeline. They agreed to work together to investigate this mystery in Flash: Rebirth #1 (DC, 2016 series).
Johnny Thunder, at 90 years old, is shown trying to find his lost Thunderbolt in an attempt to bring back the Justice Society. Wally West had appeared to Johnny in DC Universe: Rebirth #1 to tell him to find the Justice Society, who have also been removed from the timeline in the post-Flashpoint universe.
When using the Cosmic Treadmill to travel through time, the Flash and Batman view alternate past timelines of the DC Universe of which they have no memory, including the original formation of the Justice League (from Justice League of America (1960 series) #9), the JLA from during Identity Crisis (2004 series), and a dying Barry Allen appearing to Batman through time (as seen in Crisis on Infinite Earths (1985 series)).