Ultimate Avengers is an animated movie released in 2006 and the first film in the Marvel Animated Features, which was followed by Ultimate Avengers 2. It is a loose adaptation of the Ultimate Marvel comic book limited series The Ultimates that follows the origins of the Avengers and their battle against the Chitauri.

In the North Atlantic in 1945 a squadron of B-24 Liberators transport the 101st Airborne Division. The radio announces that Berlin has been taken and Nazi Germany is expected to surrender. However, there are also claims of a Nazi super weapon that is aimed at Washington, DC. Steve Rogers, better known as Captain America, leads the mission on an island near Norway where the super weapon is believed to be held. Rogers storms through the fortress and is confronted by his nemesis Herr Kleiser, who is revealed to be an extraterrestrial shapeshifter. The super weapon is launched, leaving Rogers the choice to throw himself onto the rocket and harmlessly cause it to self destruct, while having a final confrontation with Kleiser. The missile is destroyed, but Rogers falls into the ocean and the world believes he is dead.

Sixty years later, Rogers' body is recovered by S.H.I.E.L.D. director Nick Fury and thawed out into modern day. Gamma scientist Bruce Banner is working on a super soldier serum for S.H.I.E.L.D., while on medications that keep him from transforming into the Hulk. S.H.I.E.L.D. sends a satellite into space to track the three remaining alien ships, but is ultimately destroyed by the invaders, forcing Nick Fury to reluctantly initiate Project: Avenger. Fury and Rogers try to recruit the Norse god Thor, who rejects the offer to continue environmentalist protests. The resulting recruits are Captain America, Bruce Banner, Iron Man, Giant Man, Wasp, Natalia Romanov and Betty Ross. Fury briefs the team about the aliens, named the Chitauri, and the use of the metal Vibranium, while Banner struggles to repress the Hulk persona and Rogers is hesitant of leading the Avengers.

The team's first mission is to capture a Chitauri spy which has breached a S.H.I.E.L.D. facility. The Avengers disregard Rogers' orders and follow their own egos, resulting in failure which allows the alien to escape with all sufficient information about S.H.I.E.L.D.. Fury fires an arrogant Hank Pym, who is followed by his wife Wasp. Iron Man, now revealed to be Tony Stark, resigns immediately. Rogers also chooses to leave Fury, feeling unready to fight the Chitauri. Desperate for soldiers to combat the coming assault on S.H.I.E.L.D.'s New York facility, Fury demands that Banner applies his super soldier serum to all of his candidates, only for Betty Ross to learn that Banner was only using the serum as a diversion to find a way to control the Hulk. Banner steps into a "rebirth chamber", which transforms him into the Hulk, while still maintaining his own mind.

Fury and the Black Widow lead a failing attack against the alien invaders, but each Avenger shows up, now joined by Thor, to send the three ships crashing into the ocean. Banner loses control of his thoughts and the Hulk retains his own childlike and violent personality, brutally killing every last Chitauri and then lashing out against the Avengers. The Hulk proves strong enough to knock out a full-size Giant Man, damage Iron Man's suit and even, with effort, to lift Thor's hammer Mjollnir. The Avengers manage to inject the Hulk with a high dose of Banner's medication, and Betty Ross calms the Hulk long enough to let the drug render him human again. Banner is kept in a glass cell for breaking Fury's trust, and the Avengers celebrate their success against the Chitauri and praise a wounded Captain America for leading them as a team.