The League of Seven

Twelve-year-old Archie Dent, his parents, and their Tik Tok servant, Mr. Rivets, travel by submarine to the secret headquarters of the Septemberist Society in New Rome. Archie’s parents, researchers for the Society, must warn the council that the Mangleborn known as Malacar Ahasherat, the Swarm Queen, is about to break free from her prison in Florida. The Septemberist Society is a secret group that monitors the Mangleborn, ancient giant monsters imprisoned on Earth, and awaits a new League of Seven, a group of heroes with incredible powers, to fight them. While his parents meet with the council, Archie plays in the catacombs and discovers a living, spider-like Manglespawn hatching smaller offspring. He flees as Mr. Rivets crushes one of the creatures. Archie bursts into the council chamber to warn them but finds the council members, led by chief Philomena Moffett, with unnatural smiles, repeating the phrase "Jandal a Haad." He soon discovers his parents are similarly afflicted, controlled by small, bug-like Manglespawn attached to their necks.

Archie and Mr. Rivets are forced to flee with Archie’s controlled parents aboard a submersible. On the submarine, Archie has Mr. Rivets attempt to remove the bug from his mother’s neck. She briefly regains consciousness and tells him to remember "One, two, buckle my shoe." Before the bug is fully removed, Archie’s father deactivates Mr. Rivets, and his mother returns to her controlled state. They board the family airship, the Hesperus, and his parents set a course for the Swarm Queen’s prison in Florida. On the journey, Archie finds an old aether pistol and has a vivid dream of a cavern with a circular stone well sealed by a lid marked "XX." He sees millions of bugs crawling into cracks in the seal as something powerful tries to break out. The Hesperus arrives in the Florida swamps amid a lightning storm.

Archie and Mr. Rivets follow his parents into the swamp and find a clearing where inventor Thomas Alva Edison is using a lightning tower and an ancient artifact called the Archimedes Engine to channel "lektricity" into the Swarm Queen’s prison. Archie’s parents inform Edison that they know the code to bypass the prison’s puzzle traps. Archie confronts his parents but cannot bring himself to shoot them to stop them. He is then attacked by Edison's deadly Tik Tok, Mr. Shinobi, a black machine man referred to as a "meka-ninja." During the fight, Archie accidentally shoots the lightning tower with his aether pistol, causing a massive explosion that destroys Edison’s equipment and knocks Archie unconscious.

Archie awakens as a prisoner alongside a Seminole girl. Edison is furious about the destruction of his work. His apprentice, a fourteen-year-old tinker named Fergus, reveals he might be able to rebuild the Archimedes Engine. In response, Edison has Fergus’s mentor, Kano Henhawk, killed by Mr. Shinobi and captures Fergus. Archie and the girl are locked in a crate while Edison performs a ritual to turn Fergus into a living Archimedes Engine by replacing his blood with the ichor of a lektric squid. When Archie touches the green magical flame surrounding Fergus, he has a vision of the Swarm Queen and his hair turns white. The girl uses her small, winged clockwork animals to escape the crate. While she fights Mr. Shinobi, Archie pulls Fergus from the altar. Mr. Rivets arrives with the Hesperus, rescuing all three of them.

Aboard the airship, Mr. Rivets treats Fergus’s leg, which was permanently damaged by Mr. Shinobi. The squid blood has left moving black lines, like circuits, on Fergus’s skin. Archie explains the history of the Mangleborn, and he and Fergus realize they share a telepathic connection to the Swarm Queen, experiencing shared dreams of Archie’s parents navigating the puzzle traps. The Swarm Queen sends a swarm of hornets to attack the airship, and the three work together to fend them off. The girl introduces herself as Hachi, and they decide to stick together. They travel to Jersey so Fergus can inform Kano Henhawk’s wife of his death, and Fergus builds a leg brace for himself. They then go to New Rome to find another Septemberist, John Douglas, but discover his print shop has been taken over by the Mangleborn’s influence. They temporarily free John, who warns them to run, and find a scrapbook he kept detailing Archie’s entire life before they are forced to flee.

At the Pennsylvania Pneumatic Post Office, Mr. Shinobi attacks them. During the fight, Fergus is accidentally launched through a large pneumatic tube, with Mr. Shinobi in pursuit. Hachi and Archie follow in another capsule, leading to a chaotic chase through the pneumatic network that takes them through a department store and onto a rooftop. There, one of Hachi's clockwork animals, Freckles, which had gotten inside Mr. Shinobi, causes a mechanical failure that makes its arm fall off.

Escaping in their capsule, they are diverted into the maintenance tunnels beneath the city. There, they are attacked by a giant, blind, mole-like Manglespawn that hunts by seeing through its victims' eyes. Realizing its weakness, Archie tricks the creature into looking at a pneumatic tube just as a capsule fires, stunning it. They escape the tunnels and contact Mr. Rivets's last known Septemberist ally, Nikola Tesla, who directs them to his secret base, Atlantis Station, hidden behind Niagara Falls.

At Atlantis Station, a paranoid Tesla traps them in an electrified room called a Franklin cage. Fergus discovers he can absorb the electricity, revealing his new powers. Fascinated, Tesla lets them in and experiments on Fergus, determining that he can hear a homing beacon transmitted by Mr. Shinobi, who has followed them. While Tesla and Fergus work, Archie and Hachi research the Swarm Queen and learn that a hero from a past League, the Great Bear, defeated her using an invulnerable bearskin pelt. They are attacked by Tesla’s machine men, reprogramled by Mr. Shinobi. They escape by flooding the workshop, which shorts out the steam-powered machine men but not the clockwork Mr. Rivets. They trap Mr. Shinobi in the Franklin cage and escape with advanced weapons. They travel to the Great Bear Monument in Nova Scotia, where they find a hidden entrance to his tomb and retrieve a six-limbed, white fur pelt.

Aboard the airship, Archie dreams his parents are opening the final seal to the Swarm Queen's prison. Edison's airship, the Black Maria, attacks, having tracked them with a hidden beacon. During the battle, Archie uses the pelt to shield the Hesperus's gasbags, but his safety line snaps and he falls thousands of feet. Enraged, Fergus unleashes a massive electrical blast that destroys the Black Maria. The airships collide, and Fergus falls into a lake. Hachi and Mr. Rivets crash-land the crippled Hesperus. Believing their friends are dead, they set out for Standing Peachtree. Hachi reveals to Mr. Rivets that Edison was involved in the massacre of her father and the men of her mother's town, Chuluota.

Archie miraculously survives the fall and is found by John Otter, a Cherokee medicine man and Septemberist, who reveals Archie is adopted and explains the "Jandal a Haad," or "Stone Man," a powerful but dangerous strongman archetype in the League of Seven. Meanwhile, Fergus survives the crash and takes a train to Standing Peachtree, where he makes his way to Lady Josephine's Academy for Spirited Girls, Hachi's former school. Archie arrives at the academy and is reunited with Hachi and Fergus. They hide from Pinkerton agents hired by Edison during a school dance, but the agents return with a mob of local Cherokee, whose tribal tensions have been inflamed by the Mangleborn's influence. The school's students, led by Hachi, defend the academy. During the fight, Archie discovers the pelt makes him seem invulnerable, and Fergus uses his powers to incapacitate the mob. A local sheriff arrives but chooses not to arrest them, despite having a "WANTED" flier for the trio.

The trio and Mr. Rivets take a train to Florida and use the nursery rhyme clue, "One, two, buckle my shoe," to find the entrance to the puzzle traps. They navigate a series of deadly rooms based on nursery rhyme ciphers. During this, Archie has a vision of a future League of Seven fighting against him. He is separated from the others when the pelt gets caught in giant gears, causing the entire machine to collapse. They are reunited in the final chamber, where Archie's parents and Edison, his brain now housed in a mechanical body, are present. The Swarm Queen’s voice and Edison’s taunts reveal Archie is the Jandal a Haad. After surviving attacks that should have killed him, Archie realizes he is naturally super-strong and unbreakable because he is made of stone. He understands he survived the machine's collapse due to this power, not the pelt, and a physical crack appears on his arm.

The revelations drive Archie into a berserker rage, and he attacks his friends. Hachi stabs his mother non-lethally in the leg to shock him back to his senses. Now in control of his powers, Archie defeats Edison's mechanical body. As the Swarm Queen begins to emerge from her prison, Archie throws the defeated Edison into the pit on top of her. The machine to close the seal is broken, so Archie uses his immense strength to manually turn the gears and seal the Mangleborn in her prison. His parents confirm he was adopted and that the Septemberist Society brought him to them as a baby, but they did not know his true nature. The group escapes in Edison's abandoned airship. Archie accepts that the Dents are his true family. Hachi plans to hunt for "Batty Blavatsky," another person involved in her father's murder, and Fergus vows to go with her. They agree to stay together to find the remaining four members of their new League of Seven. Archie accepts his role as the League's strongman, or Shadow, and they set a course for home.