Mapes was one of the first Fremen that many of the Atreides encountered on a personal basis when they arrived on Arrakis. Mapes had approached the Lady Jessica in the hopes of determining if she was the one the Fremen prophecies spoke of. In turn, Jessica managed to learn a great deal from Mapes regarding Fremen superstitions and legends implanted on Arrakis by the Missionaria Protectiva. In true Fremen fashion, Mapes kept a crysknife concealed on her person. However, when she determined that the Lady Jessica was indeed the Bene Gesserit the Fremen legends spoke of, she gave the knife to Jessica, who kept it on her thereafter.
Shortly thereafter, Mapes was almost killed by a Hunter Seeker meant for the Ducal Heir, Paul. In return for saving her life, Mapes informed Paul that the Fremen were aware that a traitor was among them. However, she could not reveal their identity.
When the Atreides were assessing their new property, Jessica promised Mapes that the gardens that used obscene amounts of water, which the Harkonnens kept to mock the Fremen, would henceforth be held in trust for the people of Arrakis, and would not be a symbol of domination. Jessica's instructions were honored in the succeeding decades, and the gardens were maintained in the palace primarily for Fremen visitors.