DC comics official description:
Two greatest detectives in the DC Universe unite to unravel the mystery behind a certain blood stained smiley face button stuck in the Batcave wall. However, what begins as a simple investigation soon turns deadly when the secrets of the button prove irresistible to an unwelcome third party — and it’s not who anyone suspects! This is a mystery woven throughout time, and the countdown starts here!”

Along with several updates to retailers on pertinent dates for ordering issues of “The Button,” DC Comics has released a look at artist Jason Fabok’s “Batman” #21, featuring both heroes holding the iconic smiley face button from “Watchmen.”(the cover above this text)

The story explores the mystery of The Comedian’s blood-splattered smiley face button from “Watchmen” showing up in the Batcave at the end of Geoff Johns, Ivan Reis, Phil Jimenez, Ethan Van Sciver and Gary Frank’s “DC Universe: Rebirth” #1, released last year; the first time characters from Alan Moore and Dave Gibbons’ “Watchmen” have interacted with DC Comics’ DC Universe characters. Alan Moore has long made his stance clear that he doesn’t endorse DC revisiting “Watchmen” in any form.

Source: CBR.com