The plot basically is the same as the movie: Captain Jean-Luc Picard and the crew of the Enterprise-D discover the ruins of the Amargosa Observatory and send Commander Riker down to investigate.
Once there, he finds Dr. Tolian Soran, a scientist and madman bent on returning to the Nexus, a paradise dimension.
Soran escapes and the Enterprise goes on various missions to try and find him.