Roughly sixteen minutes of animated segments in The Adventures of Batman & Robin: The Video Game, for the Sega CD gaming console, are usually considered the "lost episode" of Batman: The Animated Series despite not being an actual episode. The segments are actually cut scenes, used to tie together gameplay sequences from the early-1990s video game and, as such, the sound, color and story are not of the same quality of the actual animated series. That said, the Sega CD game does play out like an episode, hence the unofficial tag.
Outside of the animated content produced for the Sega CD title, with animation provided by TMS Animation, original voice cast members of Batman: The Animated Series were also used. Plus, The Adventures of Batman & Robin: The Video Game was produced by Alan Burnett and Paul Dini, who internally referred to the game unofficially as "Mystery #86," implying and encouraging its "lost episode" status. Check out the player below to watch the "lost episode" right here!