The Mystery of the Third Planet (, Tayna tretyey planety, also translated as The Secret of the Third Planet) is a 1981 Soviet traditionally animated feature film directed by Roman Kachanov and produced by the Soyuzmultfilm studio in Moscow. It is based on a 1964 novella ill by Kir Bulychev, part of the Alisa Seleznyova children's science fiction book series.

Plot

Captain Zelyony, Professor Seleznyov and his daughter Alisa Seleznyova set out from Earth aboard the Pegas (Pegasus) starship, seeking out new animal species for the Moscow Zoo. Visiting the Moon as a resting point, Seleznyov meets his friend, Gromozeka the alien archaeologist, who advises him to turn to the Planet of Two Captains, a huge museum dedicated to Captains Kim and Buran who went around the whole universe on their ship, the Blue Seagull, for useful information on where the rare animals live. Doctor Verkhovtsev, the director of the museum, is seen visiting the moon as well; however, he avoids Gromozeka.