The film takes place after the events of the 2nd Sword Art Online anime series which covered the Mother's Rosario arc. It's an original story being written by the original light novel author Reki Kawahara, with returning staff from the TV series which includes its director, character designer and music composer.
The plot of the film revolves around a new piece of technology that takes the focus from the virtual reality online worlds of SAO & GGO into the real world using a new Augmented Reality device that has you leveling up by using real world items to defeat virtual monsters.
The name of the new game is called 'Ordinal Scale' and was made specifically for the new AR device called Augama. The idea is to have you play a game in your reality helping you increase your own fitness and health by transmitting visual, audio and tactual sensational data to you even when you're awake, unlike the previous Amusphere full-dive device of the previous games.
You can check out Aniplex's English website here for more!