In the fourth book, "The Opal Deception", Opal Koboi, who had been lying in an institution in a faked catatonic state, escapes with the help of pixie twins Mervall and Descant Brill, and begins plotting revenge against Julius Root, Holly Short, Foaly, and Artemis. Opal frames Holly for the murder of Julius Root and nearly terminates the now 14-year-old Artemis and Butler with a biological weapon. Holly rescues Artemis after being injured while escaping Opal's weapon. But Opal still seeks revenge on Holly and Artemis, so she traps them in an abandoned amusement park (comprising copies of the Seven Wonders of the Ancient World plus four extra monuments determined by the People) with only hungry trolls for company. Meanwhile, Mulch Diggums has helped Butler regain his memories and together they save Holly and Artemis in the nick of time and also return Artemis's memory. With help from Artemis, Butler, and Mulch, Holly prevents Opal from achieving her goal to uncover the Fairy People to humans.