I saw the TV version of State of Play a couple of years ago so I must have forgetten details. I also have missed the first episode. The main character in State of Play is a reporter, Cal McAffrey from Washington Globe (This is one of the changes. In the movie version, the setting is moved from the UK to the USA) - played brilliantly by Russell Crowe, who I think is one of the best actors in our time.
The movie opens with a young man running for his life, but he is caught and shot by a hired killer. A couple of seconds later, a pizza delivery guy is also shot to death by the same killer. McAffrey is assigned to write article on this double murder. Next, a congressman's lead researcher, also his mistress, is pushed in front of an approaching train. The investigation will lead to a connection among the 3 deaths, and that a big company is behind it.
I think it's great to cast Helen Mirren as the leader of the journalists. She is of the same caliber as Bill Nighy, who plays the same role in the TV series. The relationship of McAffrey and Stephen Collins's wife is not as complex in the TV series, but it must be caused by the time factor. In the movie version, a twist to the ending is added (unless I missed this part in the TV series). I can have sympathy to David Morrissey's Collins, but not to Ben Affleck's, thus I like the ending in the movie, which shows that all politicians are rotten (except Monsieur Madeleine, the major of Montreuil-Sur-Mer, and he's a fictional character). Perhaps the writer of the script is a sceptic.