The narrator of this book is Mavis Evans, whom we met in the first three books of the series. Mavis is a cat-shifter and Edie's sisters (they were adopted by the same parents). While Mavis' biological mother was a student at Mount Olympus Academy, her father is the god Hermes. In book 3 of the series ("Wither & Wound"), Edie desperately tries to save Mavis during her trial for treason. While Zeus manages to kill Mavis, Greg, a bat-shifter, takes her place in the underworld so Mavis can return to the world of the living and help Edie with the chaos created following Zeus' death. Following the events of the previous two books, Mavis is now an officer in the Underworld Reformatory, which the triumvirate of new gods (Brandee, Alaric and Zahara) has created in order to control paranormal who pose a threat to humans. There is rising tension on earth between humans and paranormals and the groups constantly clash.

Mavis has serious post-traumatic anxiety (as she admits) following her time as a prisoner at the academy, and Themis has given her special pills on which she seems to rely to calm herself. Most of all she dreads and hates the thought of her biological divine father, Hermes. Her sister is Mavis' sole comfort. Early in the book, Mavis discovers that both Edie and her vampire boyfriend Val have disappeared into a mysterious portal that opened up in Hermes' notorious club. Cassie (Cassandra), the kind-hearted seer and Greg’s girlfriend is also missing. Mavis is determined to discover what had happened to all of them. 

With the help of Val's sister, Tina, they head to Hermes' club. They discover that Cassie is in the club, yet she is heavily drugged and controlled by Hermes, as are the rest of the club's employees. Mavis turns to the trio of gods for help in searching for Edie and Val, yet they turn her down, thinking she is being too hysterical. Therefore, Mavis decides to attempt a rescue mission on her own. Tina agrees to help and Mavis decides to free some of the deadly prisoners from the reformatory to serve as their backup. All the prisoners wear special collars that control their powers (and help Mavis gain some control over them). She frees the blood-witch Tamika, Griff the bear shifter, Shauna, a pixie-vampire and Mac the incubus. Trevor, Alaric's brother, who is now a ghost, also joins their squad.

Although the squad is made of deadly inmates, in the end, they all show loyalty to each other. They confront Hermes and free Cassie and most of the employees from the club (apart from another bear shifter who perished). Following clues from Hermes, they locate Dionysus in France. Dionysus confides in them that the gods are being hunted down by a creature from Tartarus. Apparently, Hermes convinced Dionysus to use the Pandora's box device, which can open a portal to Tartarus (although Dionysus does not have the device any more). Tartarus is the birthplace of the gods and creatures arriving from there can kill them. While first opening the portal, the god Pan had died. Now, Dionysus also perishes at the hand of a mysterious dark smoke monster. The monster kills the witch Tamika as well which shocks the team.

The group realizes that whoever holds Pandora's box can summon the monster and eliminate the other gods. They head to the ruins of Mount Olympus Academy where they find Themis who seems to be going mad. She controls the smoke monster by holding its baby as a hostage. She has also incarcerated Merilee, Cassie's mother. Suddenly, Nico, a werewolf sifter appears. He is after Pandora's box for a mystery client of his. He is dragging Hermes with him and the group are using Hermes in order to fight Themis. Themis explains that if something is coming up from the portal, something else must return to it. The squad frees Merilee and the baby and as a result, the mother-monster kills Themis. Since the mother monster returned, Edie and Val should come back. Edie then gets out of the portal, yet she feels cold and her eyes are dull. Tina is nervous that Val has not returned as well so she decides to throw Hermes into the portal as well. Then Val also exits the portal, also looking numb. When Tina sees him she says "you" and then collapses.

The group is confused and meanwhile, the guardians of the reformatory (all harpies) have managed to trace their location and they are all arrested, including Mavis who is charged with treason. In the last episode, Edie visits Mavis in her prison cell, yet something feels strange. When Cassie throws an axe at Edie her body disintegrates into cockroaches and her former form disappears. Mavis understands that something bad came up from Tartarus and that the real Edie and Val are still trapped in there.

While the previous books explored Edie's relation with her father, Zeus, and Brandee's unsolved issues with her deceased mother, this book focuses on Mavis and her broken relationship with Hermes.

Pandora's box is a device that can open a portal to Tartarus. In a clear reference to the ancient myth, in which the jar contained evils, the "box" in the story also contains potentially evils, since Tartarus is not described as a welcoming place, although it is the birthplace of the gods. As in the ancient Hesiodic myth, here too Pandora's box brings ills and death. Furthermore, we may think of the formless monster as an amorphic being, similar to the "ills" which were originally contained within the box, although it should be noted that the monster turns out to be a caring mother that will do anything for her offspring. 

While at first, the gods have opened the portal using the device, encouraged by Hermes, in the end, it is the Titaness Themis who controls it (like the female Pandora who allegedly owned the box). Yet, she does not wish to bring chaos to the world, but order (according to her view at least). In the ancient myths, Themis was the titaness of law and order (see for example Diodorus Siculus, Library of History 5. 67. 3). In an ironic remark, during the discussion with the triumvirate gods in which Mavis tries to secure their help for her rescue emission, the following conversation takes place between Zahara and the advisor Prisha, "'…Pandora's box was always considered a myth.'" "'Weren’t we all?' laughs Prisha" (p. 32) myth and reality are all mixed together in this fantasy. This note may appear as the breaking of the 4th wall, the characters understand the irony of their becoming a myth themselves, yet, it also conveys the sense of "normal" reality or "real people" who suddenly find themselves in this fantastic new setting (which reference the beginning of the series). Yet, in fact, all the characters are unique in their own right.

While in the previous books the trio of new gods seemed to be the more righteous answer to the decadent evil gods, in this story they appear dysfunctional and uncaring (almost like their predecessors). It is ironic that they are called a triumvirate, or perhaps the authors wish to hint at the sinister reason behind this power pact? Yet, this trio is described a benevolent even if they struggle with growing into their new position.

The portrayal of the gods, especially the male ones, as vile and selfish continues here as well as it does in other books in the series. We know nothing of Pan (except his drinking problem). Dionysus escapes to Provence and is a wine expert, yet, his character is not fully developed. He serves as a means to propel the plot further up to the tragedy of Tamika's death.

Hermes is the main god presented in this story since he is Mavis' father. We learn that he is uncaring and even abusive. While she was a prisoner at the academy, he used to visit her only to taunt and humiliate her. Hermes is presented as a liar and a schemer, true to his role as the god of thieves and also true to some of the ancient myths which depict him as a trickster since birth (for example, his theft of Apollo's cattle as an infant, Pseudo-Apollodorus, Bibliotheca 3. 112–115).

Yet, there is another interesting aspect of the god which may be hinted at in the story, Hermes was one of the gods who endowed Pandora with gifts and then brought her to earth. In our story, he is the one who uses Pandora's box on earth, in Florida, hence he metaphorically accompanies "Pandora" fully aware of the threat she/it poses to humankind. Furthermore, Hermes opens a gate to Tartarus. While Tartarus is presented as the birthplace of the gods, it is also a kind of hell. Due to Hermes' machinations, Edie and Val somehow enter the portal and are trapped in Tartarus. In a way, we may paraphrase Hermes' role here as a psychopomp, who escorts (or sends) other souls to the underworld.

Regarding Themis, her deteriorating mind can equal the moral deterioration of the world and the loss of balance between the various species which currently occupy it. True to the female-oriented empowering message of the series, even the main villain in this episode is the female Themis. She holds the power, at least shortly.

Finally, this book offers familiar commonplaces of the misfits who share a close bonding and become an unlikely group (in resemblance for example to the Suicide Squad comics by DC Comics; the comic was created in 1959 by Robert Kanigher and Ross Andru; the modern version was created in 1987 by John Ostrander). The tension on Earth between paranormal and humans and the rise of human movement, Humans First, greatly echoes the chaotic atmosphere in the X-men universe franchise (the X-Men comics was initially published by Marvel Comics in 1963 and was created by Stan Lee and Jack Kirby). With the replacement of mutants with paranormal. Our paranormal group must face their inner as well as outer demons in order to stay alive and accomplish their mission.