Captain America: Civil War is the theird movie in its series. The first movie is basically just a backstory showing how Captain America came to be and how he was frozn in ice from the WW2 period to the present. In the second movie, the plot mainly focuses on Captain America adjusting to the new world he lives in. To add, he takes down the organization he worked for known as SHIELD. In the movie Avengers: Age of Ultron, the Avengers (lead by Captain America) fight against the evil robot Ultron. Almost losing this battle, the team is shaken immensely and arguments start to brew.
There happens to be a Civil War story arc in the comic universe. The basic plot in this is that the government tries to place restrictions on all super powered beings and a conflict ensues. Half of the avengers (lead by Iron Man) support the government act, but the other half (led by Captain America) goes against it. Other heroes and villains pick sides and a huge war between powers begins. This movie will most likely be similar to its comic book origins, but its thought that the disagreement will on a more personal level for both the Captain and Iron Man. In the second movie, Captain America sees his friend, Bucky acting as the Winter Soldier. This villain was actually a brainwashed version of Bucky, but only Captian America could experience just how strong the brainwashing was. In the new movie, there are clips showing him locating Bucky and helping him break from the immense amounst of mind control. On the other side of this, it's thought that Iron Man will find out that the Winter Soldier (Bucky) killed his parents. AS Captain America races to assist his longtime friend, Iron Man wants revenge on him and goes along with the governemnt act so he has government cooperation on finding the Winter Soldier. The Avenges will all choose sides once again and new characters such as Spiderman, Black Panther, Sharon Carter, and Ant-Man will join the conflict.