The phrase "main storyline" refers to the primary or central plot in a book, movie, or game. It is the main narrative or sequence of events that the story revolves around. Full definition
There are two main storylines in the original game and this remake adds a third parallel timeline. (darkainarts.com)
I wouldn't be surprised if there was one that added a new main storyline into the game that continues where the original storyline left off. (monamiibo.com)
It also offers a fairly lengthy and intriguing main storyline. (metacritic.com)