After a confrontation with Waverider and Rip Hunter, the villain Monarch ( Hank Hall) gains the ability to travel through time and becomes Extant. Being aided by a mysterious figure, Extant begins the systematic obliteration of time itself. Two temporal anomalies are created: one at the start of time, and one at the end of time, traveling backwards through the timestream destroying everything in their path. They will intersect once they reach the 20th century.
Heroes and villains band together to fight this Crisis in Time. The Flash is the first hero to encounter the entropy wave, in the 64th century. He tries to run through time to warn the heroes of the 20th century, but only his costume appears in our time to warn the heroes of Earth. The Time Trapper, a villain of the Legion of Super-Heroes, aids his former arch-enemies to stop the decay of the 30th century, but to no avail. In the past, heroes like Dr. Mist and villains like Vandal Savage also try to stop the decay, and fail. The Justice Society of America makes an assault on Extant, only to have Atom, Dr. Mid-Nite, and Hourman killed, along with most of the other members aged drastically. Extant is also revealed to be the secret creator of the Team Titans, and uses them as weapons against the other heroes.
Superman enlists the aide of Green Lantern's power ring to send out a call to rally all of the heroes the ring can reach. As the timestream breaks apart, alternate versions of heroes appear: a Dick Grayson from when he was Robin; Barbara Gordon from when she was Batgirl; a pre-Crisis Superboy; and an alternate history hero named Alpha Centurion. The heroes deal with their alternate counterparts, while trying to stem the tide of chaos the time rifts are creating. While the heroes think Extant is behind the crisis, the true enemy is soon revealed: Hal Jordan. Under the guise of Parallax, Jordan wanted to recreate a universe where Coast City was never destroyed.
The time rifts converge at the 20th century, destroying all time. However, the Linear Men manage to pull a small group of heroes outside of time to the Vanishing Point, just as Parallax and Extant pull themselves outside of time. The heroes regroup, and launch an attack on Parallax in the void of nothingness. The heroes defeat Parallax, and manage to restart the universe, recreating the Big Bang by using Damage as a catalyst. However, this new restarted universe varies slightly from the old one, with some discontinuities eliminated and retcons introduced in the Crisis on Infinite Earths cleaned up.
Zero Hour: Crisis in Time was a crossover event that thematically followed DC Comics’ 1985 crossover event Crisis on Infinite Earths. During the Crisis on Infinite Earths, the DC multiverse was condensed down to a single universe, which necessitated a revamping of the origins of many of its superheroes. Creative teams were engaged to revamp, or “retcon” superheroes to make their origins much simpler and to appeal to a newer audience.
After the Crisis on Infinite Earths, there were major revamps to the histories of characters like Superman, Wonder Woman, and Hawkman. However, each of these relaunches happened in varying times after the Crisis; Superman’s revamp in 1986, Wonder Woman’s in 1987, and Hawkman’s in 1989, for example. Because of these revamps, and their delays in taking effect, DC Comics’ continuity became muddled; for instance, Superman was said to never have been Superboy, which affected the history of the Legion of Super-Heroes. The retcons caused havoc in the characters’ timelines.
DC sought to rectify this by creating a new event called Zero Hour, which would relaunch all of the heroes from the same point in time. They would use this event to make some changes (minor for the most part) to characters’ backstories, to allow for previous inconsistencies. From a marketing point-of-view, they also used this event to cancel lower-selling books, while launching new books with new origins for characters. Titles that were canceled include Justice League International, L.E.G.I.O.N., Team Titans, and Valor.
The Zero Hour event took place in a five-issue miniseries (numbered in descending order from 4 to 0), and with 27 crossover books. Showcase ’94 #8-9 served as a backstory to the event, highlighting Extant’s creation. Then Zero Hour : Crisis in Time #4-0 highlights the major story arc, including Hal Jordan’s first appearance since the Emerald Twilight story arc in Green Lantern. Zero Hour: Crisis in Time #0 also featured a pullout three-page timeline of the “new” DC universe.
Each of the Zero Hour crossover books follow the same format: some event tied to the Zero Hour cross-over would happen, with the “time rift” causing the world to be wiped out. This “wipe out” was represented by pages fading to blank, and blank pages filling out the book until the back cover, with the Zero Hour logo usually appearing ion the last page. To accommodate this design, DC placed the traditional letter column earlier in the issue.
With the release of Zero Hour #0, the universe was said to begin anew. Each comic was released as a number zero, and the event was referred to as Zero Month. These comics either served as a retelling of the retconned origin of the major character or team ( Superman, Batman, Wonder Woman), as a change of direction for the character or team ( Green Arrow, Justice League Task Force, New Titans), or as a way to launch a new series altogether. The new series launched during Zero Month include Fate, Gunfire, Manhunter, Primal Force, R.E.B.E.L.S, Starman, and Xenobrood.