The U.S. government tries to rein in the Avengers after the amount of human losses and collateral damage in the first two Avengers movies and put the Avengers under U.N. control, meaning that, if the Avengers have to act against a threat, a U.N. council will have to vote to approve it first. Tony Stark is haunted by the amount of lives lost at their hands and agrees that they need to be put in check while Captain America thinks that U.N. intervention in their business will cause hesitation and, if a threat persists, they need to be able to act and act quickly. Some of the Avengers agree with Iron Man and side with him, others agree with Cap and side with him. Then there is a terrorist attack where The Winter Soldier is supposedly to blame and Captain America, sides with him to protect him and basically puts the two sides on a collision course. That's just the basics, though. The plot is actually much more complicated than that.
