Because of the events of AVENGERS: AGE OF ULTRON and a recent incident involving Captain America, the US government wants every superhero registered and under their orders. They don't want vigilantes running about. This puts Steve Rogers and Tony Stark on opposite sides and splits the group in two. Another main plot point is that The Winter Soldier has been framed for a terrorist activity and Captain knows it's bogus-and tries to find who is responsible.
I found the conflict between Captain and Iron Man a bit boring after ten minutes of that, but the movie is worth seeing for the introduction of Black Panther and most of all the introduction of the teenaged SPIDER MAN. They really need to make a new Spidey movie with this kid, the character is right on the mark.
Better than THE AVENGERS 2: AGE OF ULTRON but not as good as CAPTAIN AMERICA: THE WINTER SOLDIER.