You play a character that seems to be similar to Dr. Manhattan. The story is around 4 hours long if you are speed running, and not doing anything extra. Alfonso the developer of the game stated that he loves to speed run it, because if you aren't careful you can also accidentally destroy the cities you are trying to protect. Doing the side missions, and getting lost in the environment could surly lengthen the game for you. It is also compatible with PSVR, but can be played without it if you don't have one!
The world is 100% open to you to explore. In our interview Alfonso says that we can actually go to the bottom of the Atlantic ocean to see whats down there. Below is a gif that shows a quick shot of this.