I've only written the first half of this, just a first draft, and I go into detail with this post. So you've been warned. lol
I would start the movie out Nick Fury walking through the corridors of the Stark Mansion. He enters the lounge with the Avengers (Iron Man, Thor, Ant-Man and Wasp) recuperating, severely bruised, after a fight.(It doesn't matter who it's against, preferably Loki or one of his creations) He reprimands them for their lack of teamwork, control, and how they let Hulk demolish half of the city. Video footage is shown on a huge screen behind Fury. Fury says it was a mistake to let Stark take control, now Shield will be running the operation under new leadership. He says a Helicarrier is waiting outside to pick them up. All but Stark leave. He's stands at the window, looking out at the ocean. He goes into an aside and flashback on how they got together. Then it goes into an origin story, mostly made up of Fury and Stark scoping out a team roster, much like the Mighty Avengers 1st issue, but with comedic rejects for the team and whatnot. You find out that SHIELD is the government's way to control and police Domestic "Super-threats", another word for Superhuman activity. They've been researching the Super-Soldier Program on two fronts. The biological half has failed, been only perfected once. They are in the process of searching for that one test subject. However, Fury tells Stark that they have made a breakthrough with robotics, which he calls the wave of the future as far as a Superhuman policeforce. Stark is introduced to Ant-man and his assistant girlfriend, Wasp, who are developing nanotechnology for SHIELD, and the two inventions he's completed: Pym Particles and an advanced AI computer, U.L.T.R.O.N. (Some acronym I haven't thought of yet. Stark makes a joke about SHIELD and they're terrible acronyms). They recruit Donald Blake, a former Doctor from an insane Asylum who is convinced he is the Son of Odin sent to Earth as punishment. Hulk is Stark's recruit, and Fury is against it. But Stark says it'll be his own responsibility.
We come out of the flashback and Stark sighs by the window and follows his teammates to the HeliCarrier.
So the rest of the movie I haven't written completely, but Captain America's body is being unfrozen on the HeliCarrier, he escapes into modern society, much like the Ultimates, he's recovered and made team captain, something Stark resents him for. The whole lack of team work thing and Captain America struggling to establish it is also straight from the Ultimates. Ultron becomes sentient, finishes Hank's work on the super-robots, using one of them as a body, and the rest as his army. He takes over SHIELD's computers and finds a hidden directive, one even above Fury's pay grade. Once the robots were online they were gonna wipe out all Super-humans. Ultron acts on this, being part of his programming and takes over SHIELD. Fury, a group of special SHIELD agents (Agents Dugan, Bridge, Romanoff, Drew, and Barton) and the Avengers fight Ultron and the controlled SHIELD.