SPOILER WARNING: This article contains major spoilers for “U.S.Avengers” #2, on sale now.
We quickly learn that this is Zero Day, an event teased on the "coming this year in U.S.Avengers" page at the end of issue #1. That teaser asked "Where were you on Zero Day?" and a double-page spread in "U.S.Avengers" #2 shows where most of Earth's heroes were when the day's events came down -- they were all getting killed.
The adult Dani Cage tells the tale of how Thanos invaded Project P.E.G.A.S.U.S. and used a reality-warping Cosmic Cube to build a pair of deadly gauntlets. He then just started killing, killing heroes that look exactly like they do currently in the Marvel Universe. Dani then reveals how Thanos was defeated in her timeline, while also pinpointing when Zero Day was supposed to happen; Black Widow and a group of superhero sorcerers and scientists pooled their knowledge and a abilities, ultimately stopping the Mad Titan's onslaught.
But unlike other storylines involving time travelers from the future, Dani Cage hasn't come back in time to prevent Zero Day from happening. No, in the main Marvel timeline, Zero Day has already come and gone at this point. In fact, Zero Day, as it occurred in the main Marvel timeline, was depicted in last year's "Free Comic Book Day Civil War II" #1.
In that issue, Captain Marvel and a group of heroes got a heads up about Thanos' trip to Project P.E.G.A.S.U.S. by the new Inhuman Ulysses. Ulysses' ability to predict a likely future put Carol Danvers' team in the right place at the right time to stop an unopposed Thanos from turning a Cosmic Cube into a weapon powerful enough to destroy half of Earth's heroes. The battle, which served as the inciting incident for the recently concluded "Civil War II" storyline, cost War Machine his life and put She-Hulk in critical condition. Now we know that had the heroes not intervened, a lot more heroes would have died, leading to the ravaged, post-apocalyptic world Dani Cage comes from becoming the main Marvel timeline.
This is the first time we've seen the future where this Dani "Captain America" Cage comes from, though it's not the character's first appearance. For those unfamiliar with her, she was at the center of Al Ewing's "Ultron Forever" storyline, which spanned a number of Avengers annuals in the summer of 2015. She returned in the spring of 2016 for a few issues of Ewing's "New Avengers," where she teamed up with Sunspot's team against the evil wizard Moridun.
We then learn why Captain America has returned to this time period; Despite her reality being ravaged by Zero Day, Dani is not attempting to save her present by changing the past, as we typically see in this sort of storyline. Instead, Cage is chasing after the Golden Skull, a villain from her home timeline. Like the adult Dani Cage, Golden Skull has appeared before but his origins remain shrouded in mystery. Golden Skull served as the villain for "Ultron Forever," and this issue of "U.S.Avengers" serves up a few possible origin stories for the menace.
We learn that the Golden Skull is a bit of a mystery, with no clear origin. He could be the descendant of a traitor killed by Colonel Steven Rogers (a possible ancestor of Steve Rogers) in the Revolutionary War, the nephew of the supervillain known as the Plunderer, or the rebellious super-criminal son of an environmental activist. Dani theorizes that the Golden Skull has traveled back in time to try to loot the world on a large scale. As she explains, Dani's home time period has been ravaged by wars and "there's just not as much stuff around for him to loot." Since Zero Day didn't happen in this timeline, it's ripe for the Golden Skull's taking.
The issue concludes with the U.S.Avengers tracking the Golden Skull to a billionaire's party for other billionaires in an attempt to prevent a massive crime. Of course, the Avengers don't find out until the final pages that the massive crime has already happened; every billionaire has been replaced with a robot duplicate by the Golden Skull.
It's not yet clear what the Golden Skull's final plan is, and whether or not Zero Day will have a further impact on the storyline since it has already come and gone. There is the matter of something called "The Eternity War," which Sunspot mentioned earlier in the issue and Dani said happened later. It's possible that could be what's coming in the pages of Ewing's other team book, "Ultimates2." In that series, the cosmic being known as Eternity has been placed in chains, and the team -- including the seeder of worlds Galactus -- have been tasked with freeing him.
"U.S.Avengers" #3 arrives in stores on February 15.