The antagonist of the third book, Physik (published in March 2007), is the 500-year-old spirit of Queen Etheldredda, who is accidentally released by Silas Heap. She sends Septimus to her immortal son Marcellus Pye. He is transported back in time to become the apprentice of the young Marcellus Pye, an alchemist who teaches him about "Physik". In the present timeline of the novel, the Castle is infected by a deadly plague and Queen Etheldredda has a plan to acquire eternal life. Jenna and Nicko meet a young trader Snorri Snorrelssen with whom they travel in time to bring Septimus back, though Nicko and Snorri couldn't escape. There, Jenna is taken to the living Queen Etheldredda, but she escapes with Septimus and they return to the present Castle. Marcia then destroys the substantial spirit of Etheldredda and Septimus brews an antidote to the plague using his knowledge of Physik.