Plot summary
Eleven year old Gregor falls through a grate in the laundry room of his New York City apartment building. With his two-year-old sister, Boots, he lands in the Underland. Here giant spiders, rats, and cockroaches coexist uneasily with humans. Because a prophecy foretells that an Overlander will play an important role in the uncertain future of Underland (which is on the brink of war), Gregor's arrival is treated as no accident. When he learns his father's disappearance may have something to do with the Underland, Gregor reluctantly embarks on a life-changing adventure.
Setting
The nominal ruler of the human city of Regalia is Luxa, a girl Gregor's age, who will fully assume the throne when she comes of age sixteen. She is proud and aggressive. At first sight, Gregor believes her to be stuck-up and conceited; this feeling only increases when Boots asks her for a ball she had been playing with and Luxa will not let go of it. A royal cousin named Henry seems more friendly to Gregor, and another named Nerissa is nervous and claims to have visions of the future. Luxa's grandfather is a kindly man named Vikus, who leads a council that rules in place of the under-aged queen. Her grandmother Solovet leads the Regalian military, which includes sword-wielding soldiers mounted on large, intelligent bats. Solovet appears to have a gentle demeanor at first, but when it comes to battle tactics, she has among the finest of minds.
The Quest
Gregor, growing tired of the Underland, attempts to escape and is attacked by rats (the main enemy of the humans). Luckily, the guard Mareth and some others (Including Luxa) save his life. Gregor then learns that, according to the "Prophecy of Gray," a group of Underlanders and Overlanders (including Gregor and his sister) will set out into dangerous rat-controlled territory, in order to rescue an Overland prisoner, whom Gregor believes to be his long-lost father.
The first part of their mission involves recruiting participants from each of the various groups of underland creatures mentioned in the prophecy. First Vikus, Solovet, Mareth and others go to the cockroaches and recruit Tick and Temp (whom Boots somehow recognizes from their first encounter in the underland). They then journey to the bats, many of whom are ceremonially bonded to humans (a relationship that has both social and military benefits). After fending off another rat attack, the group goes to visit the Spinners(spiders), who promptly imprison them in a web. Luxa and her bat Aurora free the party by performing a complex aerial maneuver that shreds the web. Nevertheless, before long, the rat incursions motivate the spiders Treflex and Gox join the quest.
When fully tallied, the party includes the Overlanders Gregor, Boots and (after his rescue) their father, the Underlander humans Luxa and Henry, the cockroaches Tick and Temp (who believe Boots is a princess); the spiders Gox and Treflex (who ally themselves with the humans to fight a common enemy), the rat Ripred (a fearsome fighter and strategist whose motives are never clear), and the bats Aurora (Luxa's bond) and Ares (Henry's bond).
Boots considers the quest a grand lark, and treats all her companions as equal playmates, even though the Underland humans see themselves at the top of a rigid hierarchy. While his main goal is simply to see the quest to its conclusion so that he can rescue his father, Gregor also makes conscious decisions that challenge the dominant opinions of the Underland humans. For instance, he socializes with the cockroaches, and he defends Ripred against Henry's unjust aggression (which includes an attempted assassination).
Perils and Treachery
Of the twelve members of the party, the prophecy states that only eight will return alive. Early casualties include one of the spiders (Treflex, who dies almost immediately after joining the quest) and one of the cockroaches (Tick, who sacrifices herself in order to save Boots from attacking rats).
Deep in rat territory, the party learns that Henry -- the childhood friend and cousin of Regalia's future queen -- has betrayed the whole party to the ruthless rat king Gorger. When Gorger kills a third member of the party (Gox, the surviving spider), Gregor recalls that the prophecy mentioned that eight of the twelve would survive. Believing that his own death will assure that his father and sister will return safely to the surface, Gregor creates a distraction by plunging into a pit. In the chaos, the traitorous Henry follows him, but bats and their human riders often practice such maneuvers; Henry is confident that his bat will rescue him. In fact, however, Ares lets his bond Henry fall to his death, and instead rescues Gregor.
Resolution
Gregor's father is ill but safe. With the rat king Gorger dead, the quest is a success. Luxa is shaken by Henry's betrayal, as are the other Regalians. Nevertheless, Ares is put on trial for one of two charges: Treason (Along with Henry) or abandoning his bond. Before the Regalian authorities can pronounce Ares guilty, the quick-thinking Gregor begins to recite the ceremonial words that will create a bond between Ares and himself. Ares finishes the ceremony, and the Regalians cease to prosecute Ares.
Gregor, Boots, and their father return to New York City, but not before Vikus hints that the warrior has not seen the last of the Underland.