Medea was the Princess of Colchis, the land that held the Golden Fleece during the time of the original Jason, as well as a powerful sorceress. After the Doors of Death were opened, Medea returned to the land of the living, but was killed again. Medea was once again resurrected before the Doors were closed and allied herself with Triumvirate Holdings.

Jason Grace, Piper McLean, and Leo Valdez come to her underground mall, M's, after following some venti, where Medea introduces herself only as the Princess of Colchis. She begins to show the demigods her stock of magic objects and potions. She shows a momentary flash of anger when she hears the name of Jason, same as that of the herowho betrayed her. Soon, Piper realizes that Medea is up to no good. When Jason and Leo seem to want to keep shopping, Piper notices that Medea is charmspeaking them into buying whatever she wants them to. Piper attempts to snap them out of it with her own charmspeak and the stories of the horrible things Medea committed in the past.

Medea is revealed to have captured Gleeson Hedge and the venti (caged in the bottom floor of the mall). The demigods ask for the price to retrieve the captives. Medea says the price would be quite high since the trade is unfair, and that the person buying the product is named Jason. She continues to charmspeak them to the point where they are barely listening to Piper. Medea tells them to chop each other into pieces so she could bring them back to life even stronger (the same way she killed Pelias). Leo and Jason pull out their weapons.

Piper gets Medea to admit that she can see the future. Medea reveals that she divined that Leo would rise up against Gaea, so she warned Gaea from the Underworld. This event set in motion the awakening of Gaea and the death of Leo's mother. As a reward, Medea was set free of the Underworld and allowed to return to the land of the living, as long as her mall remained underground so Gaea could keep watch on anyone that entered.

Eventually, Jason and Leo come to their senses. Medea immediately releases her two sun dragons on the two demigods. She tries to run away and Piper chases her. Piper fatally wounds Medea by throwing poisonous chemicals on her. Meanwhile, Leo summons Festus, who helps Leo and Jason to defeat the sun dragons and pick up the cages containing Coach Hedge and the storm spirits.

Just before the demigods escape, Medea cries out that she doesn't want to be abandoned again as she was by the original Jason. She almost manages to gain Piper's sympathy by offering to cure Jason of his amnesia. The demigods choose to leave Medea, however, when they realize that she is only trying to buy more time to destroy them. She is an agent of Gaea, and the one who Hera warned them about. As they fly away with Coach Hedge and the storm spirits, they look back to make sure the she isn't chasing them.

Later in the book, Aphrodite appears to Piper in her dream where she is shopping in Medea's mall. Aphrodite warns Piper that Medea will return later on, along with others.