Taking place on one of the beta lines, Steins;gate 0 tells the story of Okabe, who after failing to save Kurisu, returns to a normal life. He is now attending college and seeing a therapist for the visions of the other world lines he has seen. He is pulled back into the past when he meets a professor from Kurisu's college who has been working on a new A.I.
It turns out that the A.I. was based off of the research Kirusu was working on with Okabe to develop the time machine. This story is meant to fill in the gaps and to bring us to the end of Steins;gate (The finding of the timeline where everyone lives). There are some familiar characters and some new ones, but we loved the original and want to see how this gets us to the end.