Avengers: Earth's Mightiest Heroes has got to be the best Marvel animated series ever made. The stories, while are taken from the comics, are not re-telling the same stories like the Marvel cartoons of 90's did.
Kang has arrived to the present because his future time-line is being destroyed by an anomaly which he found out is Captain America. He decides to eliminate the Avengers and conquer the world in the present - not only to save his time-line, but to save the live of his love Princess Ravonna. The Avengers and SHIELD are no match for Kang and his forces so Ant-man/Dr. Pym uses his guards from the Negative Zone prison to form an army. These robots are known as Ultrons and if you know Avenger history you probably know where this is going. So, after programing the robots the concept of violence, the Avengers and Ultrons attack the main ship the Damocles to stop Kang. All of the Ultrons were destroyed except Ultron 5, but the Avengers capture Kang and are trying to save Ravonna when the episode ends.
As I said, they are not doing chapter and verse from the comics, but are keeping the spirit which is great! I really enjoy this series and highly recommend it!