Brightbill leads his flock back to the island and reunites with his robot mother Roz.

A sick seal washes ashore and warns all the island animals that a deadly poison tide is approaching.

Brightbill and Glimmer fly out to investigate passing flocks and return to confirm the poison tide is real.

The poison tide reaches the island's shores and kills all the coastal vegetation and sea life.

Roz falls into a deep pond and emerges unharmed, proving her new body is completely waterproof.

Roz announces to all the island animals that she will leave to seek the Ancient Shark's help.

Roz walks north along the ocean floor and witnesses both the beauty and devastation of underwater life.

Roz locates the Ancient Shark, Gurry, who reveals that an underwater robot mining operation is causing the poison tide.

Roz persuades Gurry to allow her to negotiate with the mining station's humans before launching a violent attack.

A station drone spots Roz on the rooftop and BOSUN robots capture and lock her in a room.

Mining station manager Akiko and assistant George interrogate Roz but refuse to shut down the operation.

Gurry launches a full attack on the mining station from below while a swarm of seabirds strikes from above.

A pod of whales generates a massive tidal wave that breaches the station's outer wall and begins sinking it.

Roz rescues all the station's human workers one by one and arranges for whales to carry them to safety.

Roz returns to the island, meets her grandgosslings for the first time, and declares the poison tide is over.

The story begins with Brightbill's return from migration and the peaceful island life being shattered when a dying seal warns of an approaching poison tide. Roz leads the animals through evacuation efforts but realizes their work is not enough, and when she discovers her new body is waterproof, she decides to leave the island and find the legendary Ancient Shark. Her long journey across the ocean floor brings her to the Ancient Shark, Gurry, who reveals that a human-operated underwater mining station is the source of the poison. Roz infiltrates the station to negotiate but is captured by its robots and imprisoned by the station's manager, Akiko. Gurry's army of ocean creatures and seabirds then launches a full assault, and the whales sink the station, forcing Roz to rescue the human crew using whales as life rafts. Roz ultimately stops the mining, the cleanup crews restore the ocean, and she returns triumphantly to meet her grandgosslings and declare the poison tide over.

The key events that drive Roz's mission to save the island.

Brightbill introduces his mate Glimmer to Roz, and the two geese choose to build their own nest instead.

The island animals disagree at the Dawn Truce about how to respond to the poison tide, with many wanting to flee.

Brightbill instructs much of his goose flock to evacuate the island while he and Glimmer choose to stay.

Roz directs the island animals to dig cisterns and deepen ponds to store freshwater before the tide arrives.

A lightning storm ignites a wildfire that burns across the island's vegetation before rain finally extinguishes the blaze.

Walking the seafloor, Roz discovers an entire coastal town completely submerged beneath the rising ocean water.

A sperm whale in the deep ocean mistakes Roz for a squid, bites her, then doubts her mission can succeed.

A pod of dolphins swims with Roz and directs her westward toward the Ancient Shark's northern territory.

A puffin named Kerplunk meets Roz during a seabird feeding frenzy and points her toward a glacier to find the Ancient Shark.

Roz slips through a crack in a glacier into a dark ice cave and spends days digging her way out.

A polar bear hunts walruses on an Arctic beach, but walrus leader Big Shot steps forward to protect a small walrus.

Brightbill and Glimmer travel to the mining station and break Roz free from her prison cell with Nimbus's help.

Roz descends to the ocean floor and confronts Crusher, the giant crab-like mining robot, but cannot overpower it.

Gurry's army pushes the wrecked mining station onto Crusher, and Roz barely escapes into a sea cave to survive.

A fleet of cleanup robots sent by Akiko arrives at the island and clears the poison tide from the ocean and coastline.

The story's early chapters establish the family and community bonds at stake, as Brightbill introduces his mate Glimmer and the island animals struggle to agree on how to face the coming threat. While some geese evacuate, Brightbill and Glimmer choose to stay, and Roz organizes the remaining animals into conservation teams before a wildfire further devastates the island. On her ocean journey, Roz encounters a submerged human town, a biting sperm whale, playful dolphins, and a puffin named Kerplunk, each guiding her northward toward the Ancient Shark. She braves the glacial wilderness, survives an ice cave, and witnesses the brutal drama of walruses and a polar bear on an Arctic beach before diving toward Gurry. When Roz is captured, Brightbill and Glimmer travel to the station and help break her free, but the real challenge remains as Roz descends to face the enormous mining robot Crusher alone. Gurry's army ultimately brings the station down onto Crusher while Roz escapes into a sea cave, and the cleanup fleet Akiko promised finally restores the ocean and coastline.

The supporting events that shaped Roz's journey beneath the waves.

Roz dismisses the seal's warning and reassures frightened island animals that the ocean is too vast to threaten them.

Roz overrides her Survival Instincts and rescues a stranded otter pup just as the poison tide washes ashore.

Roz wades alone into the poisoned ocean at night and confirms the toxic water cannot harm her body at all.

At her farewell sendoff, Roz learns Glimmer is sitting on eggs, making her even more determined to succeed.

An angry sea turtle tells Roz that thousands of sea turtle eggs were destroyed by the poison tide.

Roz meets a gender-changing goby fish in a flooded town and feels a deep connection to the natural world.

Roz finds mushrooms thriving on a dead island and learns they can help heal polluted soil, giving her hope.

Roz teaches herself to navigate the deep ocean using only echolocation, switching off her headlights entirely.

Roz meets the octopus Limber, who will die after laying her eggs, and solemnly promises to protect the hatchlings.

Gurry attacks Roz to provoke her, and she instinctively punches the shark, discovering her new body can fight back.

Hidden in the control room, Roz watches Akiko cry during a video call with her partner and daughter.

Akiko and George reveal to Roz that the mining operation extracts rare minerals specifically to manufacture more robots like her.

Tiny zooplankton coat the mining station's engine propellers with thick slime, halting the station's power completely.

Roz says an emotional farewell to Brightbill and Glimmer before diving alone into the deep to confront Crusher.

Brightbill and his family take flight for winter migration as Roz races to the clifftop to watch them disappear.

The story's pivotal moments begin with Roz's early mistake of dismissing the seal's warning, a misjudgment she quickly corrects by overriding her own Survival Instincts to save an otter pup as the tide rolls in. Her decision to enter the poisoned ocean at night and confirm her immunity sets her entire mission in motion, and learning Glimmer is sitting on eggs at her farewell deepens her personal stake in succeeding. Along her ocean journey, Roz is moved by a sea turtle's grief, discovers mushrooms that could heal polluted soil, bonds with a dying octopus named Limber, and learns to navigate the deep using only echolocation. Her meeting with Gurry forces her to confront her own capacity for violence when she instinctively punches the shark in self-defense, and at the mining station, overhearing Akiko's tearful family video call gives Roz unexpected empathy for the humans she came to confront. The revelation that the mining creates more robots like Roz herself adds moral weight to her mission, and after orchestrating Crusher's defeat, Roz shares a final goodbye with Brightbill as she watches his family fly south, accepting her new purpose as protector of the entire island.

The discoveries and decisions that transformed Roz's understanding of her world.