The last time this many Avengers assembled it was to defeat the nearly indestructible android Ultron in "Avengers: Age of Ultron."
So what was the dire threat this time?
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"Avengers" director Joss Whedon called upon a few of his celebrity friends, 27 in all, in a video aimed at encouraging voter registration and keeping Trump out of the White House.
The video, released by activist production company Save The Day, features 27 celebrities ― including Robert Downey Jr., Scarlett Johansson, Mark Ruffalo, Julianne Moore, Neil Patrick Harris, Keegan-Michael Key, Don Cheadle, James Franco, Taran Killam, Stanley Tucci, Martin Sheen, and Leslie Odom Jr.
With both humor and earnest conviction, the stars laid out their case why no one should sit on the sidelines in the upcoming election.
"You only get this many famous people together if the issue is one that truly matters to all of us. A disease, or ecological crisis or a racist, abusive coward who could permanently damage the fabric of our society," the stars said.
"We cannot pretend both sides are equally unfavorable," says Sheen.
"We can't say one vote doesn't matter," says Blindspot star Ashley Johnson.
"Your vote matters," added Cheadle.