Mind-controlling parasites are travelling back in time through memories to destroy all humans at the source. There is one in your brain and you must not let anyone know or super soldiers from the future will murder you. This is a game for two people. One of you will play the infected human, and one of you will play the forces around that human. We'll refer to these two players as the Human and the Worm.
Worm, you take the Human's responses to the questions and use them to start a scene. Describe where the Human is, what and who is around him, and answer any questions he asks. Human, you take in what the Worm says to you and respond to her by describing what you do. If the Human's character talks to people, the Worm will pretend to be the people. In other words, you just make up what you're doing (Human) and what's going on in the world (Worm). It's all pretty free form; you're making things up and seeing what happens. The Human's character will work toward finding The Traitor and avoiding Lantern Men. And the Worm will put obstacles in the Human's way and react to the Human's choices.