SPOILER WARNING: The following article contains spoilers for Doomsday Clock #2 by Geoff Johns and Gary Frank, on sale now.

Last year's DC Comics: Rebirth #1 had posited a surprising and previously unsuspected connection between the DC Universe and Alan Moore and Dave Gibbons' lauded Watchmen series. Since then, the Rebirth initiative has also reestablished certain characters and stories as part of continuity that many believed were gone with the onset of DC's New 52 relaunch. Geoff Johns and Gary Frank's Doomsday Clock #2 further explores both of those aspects in an unexpected way.

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The DC Universe/Watchmen association is further deepened by way of the first-ever meeting between characters from both worlds. Along the way, Doomsday Clock #2 also gives a quiet, understated nod to another classic DC story by Moore; namely, his and Brian Bolland's Batman: The Killing Joke. Moore and Bolland's controversial Batman story, in fact, was released only a year or so after Watchmen concluded in 1987.

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In Doomsday Clock #2, Ozymandias (Adrian Veidt) and the new Rorschach – whom Veidt addresses for the first time as "Reggie" – leave their world in search for their former colleague Doctor Manhattan. Veidt and Rorschach believe that Manhattan is the key to saving their planet, which stands on the verge of nuclear destruction. It appears that the pair might be too late as nuclear missiles approach, but regardless they safely make their way to the world they believe Doctor Manhattan has traveled to.

Where Have We Seen That Funhouse Before?

That world, of course, is Earth-0 – center of the DC Multiverse and home to familiar figures such as Batman and Lex Luthor, both of whom are seen in this issue. The commandeered Owlship crash lands at a locale that might seem familiar to readers of The Killing Joke: an abandoned amusement park that looks eerily similar to the one predominantly featured in Moore and Bolland's graphic novel. That story, of course, was largely set in such a park that the Joker used to psychologically torture Gotham police commissioner Jim Gordon.

As Doomsday Clock heavily leans on characters and concepts created by Moore, the appearance could simply be a nod to another of Moore's best-recognized DC works. The common belief, though, holds that Moore likely considers Doomsday Clock as anything but an honor to his original Watchmen story. An homage to Killing Joke in that context makes the amusement park reference may seem more like an insult than a tribute to the creator, so it's likely that its appearance holds deeper significance than as a mere Easter egg.

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While it's possible that Gotham has more than one shuttered amusement park within its boundaries, this one's physical similarities make it easier to believe that it's the same park as that used by the Joker. The facility's characteristics drive a larger meaning to its significance than as a continuity footnote, however. Its appearance here postulates that The Killing Joke is in fact a story that's remaining of history within Rebirth continuity. Since DC Comics: Rebirth #1 dropped last year, rampant speculation has centered around which past stories remain official canon. Doomsday Clock #2 proves that one of DC's most notorious stories have officially survived into Rebirth.

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Yes, There Was An Oracle in Rebirth

Yes, this fan-favorite graphic novel remains a "real" story, but it also establishes that the story's significant elements are firmly entrenched in the DC mythos. Moore and Bolland's story presented the origin of the Joker – or at least one of them, if the multiple Joker theory proves to be true – and has since become recognized as the character's definitive backstory. Perhaps more importantly, the horrible physical and emotional trauma inflicted by the Joker on Barbara Gordon also remains as part of her history.

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Of course, this means that her time as Oracle has also survived into Rebirth. Barbara Gordon's reintroduction in DC's "New 52" in 2011 reestablished her as Batgirl, but called into question The Killing Joke's standing as canon. DC's editors assured its place in continuity, but her transition back to Batgirl largely went unexplained, and remains so well into Rebirth. Whatever the nature of her recovery proves to be, Doomsday Clock shows, not tells, that the circumstance of her injuries remains in place. Her resumption of her role as Batgirl, though, stands to be one of the many stories yet to be told in Rebirth.

It Was the Darkest of Times

If there is any acknowledgment of The Killing Joke on a more visceral level, it's one that speaks to changing mood of comics at the time. Watchmen was a dark examination of the superhero mythos that saw publication at the dawn of the so-called grim 'n gritty era. The even darker and more exploitive Killing Joke further explored a shadowy take on a classic hero vs. villain dichotomy. The amusement park reference is akin to an extension of the darker theme – Doomsday Clock not only continues the same vibe established in Watchmen, but also harkens back to the tone of the era in which Moore and Gibbons' seminal classic was initially published.

Eventually, the backlash against the ever-growing tide of darker and grimmer comics created a shift towards other gimmicks intended to grow readership. With a tale that takes place only seven years after Watchmen, though, Doomsday Clock reads like a straight-line extrapolation of where comic stories were heading 30 years ago. The reference to The Killing Joke serves in that regard serves as a marker, staking a spot in the mindset of which the story is anchored.

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Doomsday Clock #2 shows that the series might hold a larger role than merely explaining the relationship between Watchmen and the DCU. Rebirth has shown that elements of DC continuity thought to be gone might just stand to return, but which ones remain might need further explanation elsewhere. Where Killing Joke fits has now been touched on, but plenty of other past stories might stand to benefit from a similar nod.