This issue is a variant of The Flash (DC, 2016 series) #22 [Jason Fabok Lenticular Cover]. There exist further variants.
Cover is an homage to the cover of Flash Comics #1 (1940), featuring a modern version of Jay Garrick burning through the original cover of Flash Comics #1.
Story continued from Batman (DC, 2016 series) #22.
Story credits include special thanks to Geoff Johns and Tom King.
Professor Zoom was shown to have died in Batman #21 after encountering a unknown entity. Here Bruce and Barry arrive in the time stream prior to Zoom's death to witness it, although they do not know who has killed Zoom.
Jay Garrick tells Barry that he has been lost in time as was the original Wally West. Barry was able to rescue Wally and bring him back to the real world in DC Universe: Rebirth #1 (DC, 2016 series). However, when Barry tries to pull Jay back to reality, Jay is pulled away by an outside force.
At the end of the story, we see the Button, which Professor Zoom was carrying when he was struck down, being retrieved by a blue hand, presumably Doctor Manhattan from Watchmen (DC, 1986 series). Doctor Manhattan presumably was one of the unidentified speakers at the end of DC Universe: Rebirth #1 (DC, 2016 series).
In this story's epilogue, we see the Button apparently morph into the "S" shield on a Superman costume. This storyline continues in Doomsday Clock (DC, 2018 series).