For hundreds of years, sea monsters have surfaced to wreak havoc against sailors and societies
established inland.
To counter this, sailors venture outward on their ships to hunt the beasts across the
ocean, the most famous of these being the crew of "The Inevitable", led by the legendary
Captain Crow, his first mate Sarah Sharpe, and his adopted son, the equally unrivaled
monster hunter Jacob Holland.
They are financially backed by the King and Queen of the Crown, who have established a
distinguished society dubbed "Three Bridges".
After being nearly killed during a hunt, Crow tells Jacob that he will make him captain
once they kill a beast known as the "Red Bluster" that took his eye, which will also serve as
Crow's last hunt.
The crew returns to Three Bridges to collect their payment for their latest catch but are
told by the King and Queen that they will soon be replaced by a naval vessel dubbed
"The Imperator", headed by Admiral Hornagold, who will carry on the hunting of sea beasts
in their place.
Jacob proposes that his crew be given one more chance to kill the Red Bluster to continue
hunting beasts in their name should they succeed, which the King and Queen accept.
Moments after they depart, the crew discovers that an orphaned girl named Maisie Brumble,
whom Jacob met the night before, has stowed away on the ship to join them, having been
inspired to do so by her late parents who died during a hunt, and is a big fan of Crow,
Jacob, and Sharpe.
One night, the Inevitable is attacked by the Red Bluster.
Despite Crow's orders, Jacob hesitantly allows Maisie to cut a line connecting the ship to
the Bluster and saves the crew but the Bluster slips away.
Angered, Crow holds both of them at gunpoint and demands Jacob bring Maisie to him, before
the Bluster emerges from the depths and swallows Maisie and Jacob whole.
Stuck inside the beast, Jacob and Maisie escape through the Bluster's nasal cavity where they
wind up stranded on an island populated by several beasts, the Bluster among them.
Maisie discovers that the Bluster doesn't react with malice towards the both of them
and begins to befriend the beast, naming her "Red".
Maisie begins to believe that the monsters they were hunting are misunderstood creatures,
which Jacob denies.
The next morning, she befriends a smaller creature she names "Blue", during which she
and Jacob disturb a large nest of infantile beasts, forcing them to escape the island.
When a large crab-like beast tries to kill them, Red emerges and defeats it, saving them.
Jacob and Maisie are able to convince Red to take both of them (including Blue) to Rum
Pepper island so that they can secure a ship to take them back to Three Bridges.
Meanwhile, believing Jacob to be dead, Crow slips into madness over his desire to kill
Red.
He seeks out an elderly merchant named Gwen Batterbie, who gives Crow a poison powerful
enough to kill Red and a massive weapon capable of firing a harpoon tipped in it.
While on Red's back, Jacob and Maisie bond with the creature and each other.
Jacob grows an epiphany and supports Maisie's belief of the beasts being innocent, renouncing
his ways as a hunter.
They reach Rum Pepper Island but discover that the Imperator and Hornagold are stationed
there.
Red attacks the vessel, which rains hail fire at the beast with cannons, one of them wounding
Maisie.
Before the beast can kill Hornagold after she destroys the Imperator, Jacob momentarily
stops her from doing so.
Red spots the Inevitable and attacks, but is shot by the poison-tipped harpoon and nearly
dies, with Crow keeping her alive long enough to bring her to the Crown as a trophy.
Maisie is nursed back to health but is imprisoned in her room aboard the Inevitable as it arrives
to the Crown, with Red in tow.
After Blue returns to free Maisie, she discovers that all the books she's read about the beasts
and their hunters are propaganda created by the Crown.
In front of a large gathering of civilians in the kingdom, Crow prepares to publicly
execute Red before being stopped by Jacob.
Crow overpowers Jacob in a vicious duel, while Maisie and Sharpe, who begins to believe Maisie's
worldview of the beasts as well, free Red from her binds.
Maisie and Jacob convince Red to spare Crow, and expose the Crown for their lies and deceptions
and the kingdom renounces their beliefs about the beasts, including Crow.
With Red and the other sea beasts left alone, Maisie and Jacob begin their new lives together
as a family, alongside Blue.