The Purple Ball.

Alisa Seleznyova and her father professor Seleznyov are traveling in space. They meet their old friend archaeologist Gromozeka, who's just discovered a planet all inhabitants of which died. It became known that they discovered a virus of hostility, got infected and killed each other. Gromozeka also discovered that they had left the virus on Earth 26000 years ago, and the virus is about to become loose. The only chance to save the Earth is to travel 26000 years back in time - to the epoch when witches, dragons and magicians lived along with usual people.