Wandavision is a 6-episode limited series. A few are speculation on my part, but the show centers on Wanda whose powers have grown stronger after Avengers: Endgame, seemingly able to warp reality now. Either she is having a mental breakdown or she is being manipulated, Wanda resurrects Vision and transforms a town where the couple is now married and living the ideal Americana sitcom setting from the 1950s to 80s(?). Witches might be involved as evident with the presence of actress Kathryn Hahn, who is likely playing Agatha Harkness. The situation in the town has gotten worse as it draws the attention of S.W.O.R.D (Sentient World Observation and Response Department) whose members include Monica Rambeau (the girl from Captain Marvel), Jimmy Woo (Ant-Man and the Wasp) and Darcy Lewis (Jane Foster's assistant in the Thor movies). The events of Wandavision will directly tie-in to the upcoming 2022 movie Doctor Strange in the Multitude of Madness (Nightmare could be behind Wanda's breakdown).