Mantis was raised by the living planet Ego from when she was young. (According to the film producers.)
The Guardians of the Galaxy members Gamora, Drax the Destroyer, Rocket Raccoon, and Groot encounter Mantis where she takes them through the tunnels beneath the Spartaxian Royal Palace in order to save Star-Lord. They go through various obstacles where Mantis tells them to believe. After the group frees Star-Lord, they end up in the throne room where the Cosmic Seed energy is even after a fight with the robots there and a brief fight with Emperor J'son. When Mantis claims that J'son is a tyrant, she tries to attack him only to be subdued by the Guardians of the Galaxy. While being remanded to the dungeon, Mantis still claims that J'son is a tyrant and that they will eventually believe that fact.