Looks like the events of Avengers: Age of Ultron and Captain America: The Winter Soldier have come to this.
Looks like Bucky Barnes; the childhood friend and protector to pre-Super Soldier serum Steve Rogers, wartime companion of Captain America, fallen member of the Howling Commandos, and former cybernetically enhanced - mind controlled - cryogenically frozen legendary cold-war assassin called the Winter Soldier is in trouble. Folks think he's still a bad guy after escaping the control of HYDRA.
Cap is his friend and he's come to help even if that puts him at odds with the new rules that govern folks who act as 'Super Heroes'. Seems that in saving his best friend the Captain is pulling his teammates in two directions - stick with the law or stay loyal to your friends. Iron Man is leading the other side hoping that his friend will come around and keep the peace.
So that's my run down and thoughts... so maybe I'll show off the trailer. Be warned that the fight sequence at the end of the trailer looks simply epic.