The story follows Tony Stark (Iron Man) and Bruce Banner (Hulk) as they attempt to launch a dormant peacekeeping program called "Ultron." However, things go horribly wrong when the self-aware artificial intelligence concludes that the most effective way to save the planet is to extinguish humanity. The Avengers must assemble to stop Ultron, a technological threat bent on human extinction, while facing their own internal conflicts and the introduction of powerful new entities.
If you found this article via the "123 Movies" search, you are likely a Marvel fan planning a marathon. Here is the correct order, so you don't watch Ultron out of context: avengers age of ultron 123 movies