It starts out at the end of WWII were Captain America is leading a strike force against the last vastages of known Nazis, where its revealed that the Germans were in cohoots with shapechanging aliens called Chitari, who are the Ultimate Universe equivilent of the Skrull(from Secret Invasion fame). They plan on launching a nuke on America, but Cap manages to climb on the missle and stop it. However, he ends up falling into the North Pole and remaining frozen for decades. Cut to modern day where "General" Nick Fury(the Samuel L. Jackson version)is leading SHIELD on an expedition to find Captain America's body so that they can use his DNA to synthesize the Super-Soldier Serum in their constant feud with the Chitari. Cap is however not dead, and decides to join SHIELD as part of their new Project Avenger team which Fury is assembling to protect the Earth from those obnoxious aliens. They recruit Giant Man, Wasp, and the mysterious Iron Man, along with SHIELD operative Black Widow(rroawr!), although Thor, the self-proclaimed Norse god, rejects them. Dr. Bruce Banner is the reproducing the Super-Soldier Serum, but mainly wants to do it to control the raging spirit that dwells within him which is the Hulk, a monster he turns into from when he first tested the serum on himself. After failing to stop the Chitari infiltration of a military base, the team disbands. But they all manage to come together to stop the Chitari from destroying the main SHIELD headquarters, and then have to then take down the Hulk who goes all "Russel Crow" on everyone. After finally turning Hulk back to normal, the rest of the heroes sign on to be a permanent team called The Avengers(even though in the comics they're called The Ultimates).
As far as a made-for-video feature goes, this was pretty bad-ass. It has acceptable animation, even though its not theatrical quality, and a good selection of voice actors. I'll admit that I was hoping they'd take some of the more mature aspects of the actual Ultimates comic, like Hulk going on a rampage because Betty Ross dates some actor douchebag, and Giant Man using ants to attack Wasp. But since this movie was meant to appeal to a wider audience, most of that material was left out. I'd say this is probably the best superhero team movie ever done(team, not superhero group like X-Men or Fantastic Four), mainly for the way the characters put aside their personal differences to come together and defeat their common foe in the end. This was followed by the equally fine sequal, Ultimate Avengers 2, and supposedly the totally odious movie of The Invincible Iron Man was a prequal to it. BTW, there's no connection between this and the other Marvel animated movie, Next Avengers, which was more of a What If? story involving the classic Avengers' kids. I'd say that even if you're a moderate fan of superheroes or comics, that you should give this one a look, at least for some "amazing adventures, True Believers"!