At 14, Oscar Pill is no ordinary teenager. Part of the Medicus Order, he has a special gift: he can travel inside the human body and its mysterious spheres. But these days, Oscar's thoughts are elsewhere: he's fallen in love for the first time ever. Lovely Tilla is completely unaware of Oscar's gift and can't understand his strange disappearances, while he is desperate not to let her in on his secret.
When eleven students from Oscar's school in Babylon Heights go on a class trip to Paris with fellow teenagers from around the globe, some of the Medicus accompany them. The Pathologicus, arch enemies of the Medicus, are out to destroy mankind by unleashing terrible epidemics. They've declared war, and danger looms. The Medicus need all the members they can get, and go to Paris to help prepare Oscar and his friends for combat.
As Oscar and his classmates discover the French capital, they also learn how to perfect their powers within the third sphere of Embrye, the world of the womb, and target for the Pathologicus's next attack In a series of gripping adventures, Oscar and his classmates do their best to learn how to navigate this new sphere whilst fighting the Pathologicus within the human body and outside – from the shadowy corners of the Louvre and the top of a bewitched Eiffel Tower , Oscar and his friends strive to keep the Pathologicus at bay.
Oscar also discovers the most unspeakable truths about his father, his past and his destiny. But for this special teenager, the choice he'll have to make will be far more painful than these revelations: must he renounce his first love in order to save the world?