Pandemonium is the second installment in Oliver’s Delirium trilogy, and an understanding of the first novel’s world, key terms, and events is necessary to follow the sequel. Delirium is set in a dystopian version of the United States where love has been classified as a disease, amor deliria nervosa, and the government mandates a surgical procedure known as the “cure” for all citizens at age 18. The procedure eliminates the capacity for intense emotion, rendering the cured population passive and obedient. Society is tightly controlled: Cities are enclosed by electrified fences, citizens carry identification papers, boys and girls are kept rigorously segregated, and a sacred text called The Book of Shhh provides the ideological foundation for the regime. The unregulated territories beyond the fences are known as the Wilds, and the government represents them as uninhabited and dangerous, reinforcing public fear of life outside state control.

The novel’s protagonist, 17-year-old Lena Haloway, lives in Portland, Maine, with her Aunt Carol and awaits her cure. Lena has been shaped by personal tragedy: She was told her mother died by suicide after three failed cure attempts, a story that has made Lena especially fearful of love. During her pre-cure evaluation, a protest staged by Invalids (people who live uncured in the Wilds) disrupts the proceedings, and Lena notices a boy laughing from an observation deck above her. She later learns his name is Alex Warren and discovers that he is not cured, as his faked procedural scar suggests, but is an Invalid living undercover in Portland and working for the resistance.

Lena and Alex begin a forbidden relationship that alters Lena’s understanding of the society she lives in. She reconciles with her best friend, Hana Tate, after they drift apart over Hana’s increasing defiance of government rules, and Lena visits the Wilds with Alex. She also makes a significant discovery: Her mother did not die but was imprisoned for over a decade in the Crypts, Portland’s underground prison, before eventually escaping into the Wilds. This revelation undermines Lena’s trust in the regime and leads her to flee to the Wilds with Alex.

Their escape plan is discovered, however, and Lena is restrained by her family while her cure is moved up. Alex rescues her and the two race for the border fence, pursued by regulators and guards. In the novel’s final scene, Lena manages to climb the fence and cross into the Wilds, but Alex is left behind, overwhelmed by the security forces closing in on him. Lena’s last glimpse of Alex is of him standing behind a wall of smoke, bleeding and surrounded, and she enters the Wilds believing he is dead. Pandemonium opens in the immediate aftermath of this crossing, following Lena as she struggles to survive alone in the wilderness and is eventually found by a group of Invalids led by Raven.