"Tayna Tretey Planety", also called "The Mystery of the Third Planet", is a Sci-Fi/Cartoon movie directed by Roman Kachanov in 1981. It is produced by an animation studio called Soyuzmultfilm.
Basic plot: In the late 22nd century, a little girl named Alisa, her father professor and a pilot are on a space mission to find some rare animals for their Moscow Zoo. Instead for what they are supposed to do, they stumble on a case which involves Doctor Verhovtsev, who holds a conspiracy against two legendary captains. And so they are given a chatter-bird, which is the only clue for this mystery...
I recently finished watching this film in Russian, before writing this review. As I have already watched multiple animated films from Soyuzmultfilm, I have to confirm this as one of the best animated movies which were made in the Soviet Union. For something made in 1981, the traditional animation is very typical for something made in the same country, and the setting is pretty similar to various Science Fiction/B-grade movies which were made during the 60s to late 70s. You may even be hooked both by the animation and how the plot unfolds itself, like I was. The background music is pretty great as well. This 48-minute long film is a great movie to be watched by whole families.
Are you into Russian animation, as well as anything set in future and space, then this animated movie is really worth to find and watch. In my opinion, I think this film deserves an overall rating 9/10.
Some other animated films from Suyuzmultfilm I'd like to recommend, include: "Maria, Mirabella" (1981), the "Maugli" film-series (1967-1971 and the 1973-movie), and "The Humpbacked Horse" (both the 1947- and 1975-version).