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The following contains spoilers for The Flash, now playing in theaters.
A new live-action version of Supergirl appeared in 2023's The Flash due to Barry Allen's time-traveling antics. The Flash inadvertently created a dark new timeline after meddling with the past in an attempt to save his mother's life. When he accidentally arrived in his new timeline early, he discovered that key events had changed while others proceeded according to his memory.
Kara Zor-El arrived on Earth instead of her cousin Kal-El, and she joined Barry Allen as Supergirl to take on General Zod. Unfortunately, Flash and a younger version of Barry Allen watched her die countless times as they tried to alter the timeline. The tragic deaths called back to both Flash and Supergirl's original deaths during the Crisis on Infinite Earths comic event, which forever changed the DC universe as fans knew it.
Sasha Calle's Supergirl Met Her Match With General Zod In The Flash
General Zod arrived on Earth, looking for the last Kryptonian in The Flash's new timeline. Unfortunately, Superman never arrived to defend Earth from Zod and his army in the timeline Barry Allen created. The Flash recruited Michael Keaton's Batman to help him free the new timeline's Kryptonian from a government agency. While Barry Allen expected to find Kal-El, they instead freed Kara Zor-El. She agreed to help Flash and Batman take on General Zod and stop them from terraforming Earth onto a new Krypton. Unfortunately, years of imprisonment coupled with her inexperience left her vulnerable when facing powerful Kryptonian warriors like General Zod and Faora.
Both Michael Keaton's Batman and Sasha Calle's Supergirl died in battle, though the Flashes ran back in time to give it a few more ill-fated shots. Unfortunately, the eldest Barry Allen realized that they couldn't stop this moment from happening without going back even further to stop himself from changing the timeline in the first place. While The Flash loosely adapted the Flashpoint comic event, Supergirl's death left an impact on the new DC timeline much like it did in 1985's continuity-altering Crisis on Infinite Earths maxi-series by Marv Wolfman and George Pérez.
Barry Allen Sacrificed His Life To Save Earth And Warn DC's Heroes
DC's multiverse existed alongside the anti-matter universe, ruled by a powerful cosmic being known as the Anti-Monitor. He soon set his sights on expanding his anti-matter realm into the positive-matter universe by destroying every reality one by one. The Monitor was the positive counterpart to Anti-Monitor's evil, and he observed the development of the DC multiverse for centuries. When the Anti-Monitor began his devastating attack, the Monitor began assembling a collection of heroes and villains to help fight back against the Anti-Monitor.
Barry Allen had been living a retired life in the future with his love, Iris West-Allen. The Anti-Monitor's actions threatened to destroy Barry Allen's idyllic future, so he suited up once again as The Flash and sped into the past to investigate. The Anti-Monitor found and imprisoned Barry Allen, though he escaped in time to save the world. The Anti-Monitor powered up a powerful anti-matter cannon capable of destroying Earth in one fell swoop. The Flash moved faster than he ever had before and dismantled the cannon before it unleashed its fatal blast, though his body began to disintegrate in the process. With his final bursts of speed, The Flash delivered a series of warnings to other heroes across the timeline that helped prepare them to continue the fight against the Anti-Monitor.
Barry Allen's sacrifice foiled Anti-Monitor's initial plans to destroy Earth, though it only slowed down the dedicated conqueror and left DC's heroes without one of their strongest warriors. Characters from across DC's vast multiverse united to take the fight to the Anti-Monitor and stop his ongoing eradication of parallel realities.
Supergirl Died Fighting The Anti-Monitor During Crisis On Infinite Earths
One of the best versions of Supergirl appeared during DC's Silver Age, years before the iconic Crisis on Infinite Earths event. Kara Zor-El was Superman's cousin from Krypton who arrived years after Kal-El had already grown to adulthood. She became Superman's secret weapon, though she still developed a close relationship with heroes like Barbara Gordon's Batgirl. Supergirl joined the alliance of powerful DC heroes gathered by The Monitor alongside her cousin, as well as the Golden Age versions of Superman and Power Girl. They tried to take the fight to the Anti-Monitor on the anti-matter planet of Qward shortly after The Flash's sacrifice to save Earth.
During the epic battle, the Anti-Monitor got the drop on Superman and threatened to take out the Man of Steel. Supergirl intervened, destroying Anti-Monitor's machines before attacking the powerful villain with all of her might. Unfortunately, an untimely distraction by the new hero Dr. Light cost Supergirl dearly. Anti-Monitor delivered a final fatal blast of energy to cover his retreat while Superman watched helplessly. Supergirl succumbed to her extreme injuries in her cousin's arms, which fueled the Last Son of Krypton in the final battle against the Anti-Monitor.
Flash And Supergirl's Deaths Heavily Impacted The New DC Universe
Supergirl and Flash's deaths had very different impacts on the DC universe, given the reality-altering climax of the Crisis on Infinite Earths event. Following the defeat of the Anti-Monitor, the DC multiverse was reborn as a singular universe with a heavily revised timeline. While Superman grieved tremendously for his lost cousin during the Crisis, he didn't even know she existed in DC's new timeline. Kal-El was once again the only survivor from the planet Krypton, which would last for decades until a new version of Kara Zor-El appeared shortly before 2005's Infinite Crisis event.
As for The Flash, his absence left an even greater hole in the DC universe. Young Wally West/Kid Flash joined DC's heroes in their attack against the Anti-Monitor on Qward. His mentor Barry Allen appeared to him during one of his warnings across the timestream. He led Kid Flash to discover the remains of Flash's costume where it fell after his death. Kid Flash kept the costume and decided to keep Barry Allen's legacy alive by taking over as the new Flash. Wally West spent years in the role of The Flash, discovered the Speed Force, and built up the Flash Family until Barry Allen's eventual return following the Final Crisis event.
While Wally West succeeded Barry Allen as The Flash, even he struggled with the daunting task of measuring up in his mentor's shoes after the hero's sacrifice during the Crisis. A new character eventually replaced the original Kara Zor-El as Supergirl while her cousin forget she existed, though Barbara Gordon occasionally dreamed about a blond girl she couldn't help but miss for some unknown reason. Even with the reality-altering confusion caused by the Crisis, the deaths of Supergirl and the Flash left heroes devastated and changed the DC universe forever.