The tale unfolds in a swampy bayou, where we initially meet a young girl aboard an abandoned riverboat. In an innocent moment, she drops a bottle into the water, which begins a journey that will cross miles and lead to unforeseen adventures. As time passes, this bottle washes ashore in the bustling metropolis of New York City, where a group of inquisitive mice stumble upon it.
The bottle is promptly presented to the International Rescue Aid Society, an elite assembly of mice who gather within a luxury suitcase tucked away in the basement of the United Nations building. Among them is the astute janitorial mouse named Mr. Bernard. Upon retrieving the message within the bottle, the organization’s Chairman and a dedicated Hungarian representative named Miss Bianca scrutinize the heartfelt plea within the note:
> "To Morning Side Orphanage, New York
I am in terrible trouble. Please hurry. Help!
Penny."
With determination in her heart, Miss Bianca offers to assist the distressed young girl, and to everyone’s surprise, she selects Bernard as her companion on this urgent mission. Taking clues from the letter, the pair sets off to Morning Side Orphanage. Once there, they dig into a closet where they discover a box with Penny's name written on it, containing a collection of her treasured belongings. As they reflect on what could have happened to her, they are interrupted by the genial presence of Rufus, the friendly old cat residing at the orphanage.
Bernard interrogates Rufus to glean more information about Penny's disappearance. The old feline reveals that before she vanished, Penny had expressed a deep yearning to be adopted and find a loving home. Sadly, the police have ceased their search due to a lack of leads. When Bernard presses further, Rufus mentions a “trashy lady” who offered Penny a ride in her vehicle. This woman and her associate are known to operate a pawn shop just down the road from the orphanage, prompting Bernard and Bianca to investigate this lead.
In a flash, our tiny heroes find themselves at Madame Medusa’s Pawn Shop, where they uncover a writing book inscribed with Penny’s name. However, their discovery is short-lived as Medusa makes a sudden entrance to answer a phone call. Eavesdropping, they overhear her berating a worker named Snoops for failing to locate a prized diamond and for letting Penny send out her messages in bottles. Enraged by the incompetence displayed, Medusa announces her immediate plans to head to Devil's Bayou, and in a stroke of fortune, Bernard and Bianca manage to stow away in her suitcase.
However, chaos ensues when Medusa’s reckless driving causes their luggage to spill onto the sidewalk, hindering their progress. Not to be deterred, the clever duo swiftly packs another suitcase and climbs atop a skyscraper rooftop, hoping to catch a flight to Devil's Bayou aboard Albatross Airlines, piloted by an albatross named Orville. After a wild take-off, they embark on their aerial journey toward the bayou.
Upon reaching the bayou, they encounter a series of challenges, including dodging an array of fireworks set off by Mr. Snoops, all to assist Medusa in her quest to retrieve Penny. As they navigate the swamp, they meet two muskrats, Luke and Ellie Mae, who assist Orville after he is grounded due to an unfortunate mishap that singes his tail feathers. It is Ellie Mae who introduces them to a dragonfly named Evinrude, who becomes pivotal in navigating through the murky waters.
The action escalates when they discover that Medusa’s alligators, Nero and Brutus, have captured Penny and brought her back to the riverboat. Once Medusa returns, she angrily chastises Snoops for his lack of results and explains to him their nefarious plan: that Penny was kidnapped specifically because of her small size, allowing her to retrieve a giant diamond known as The Devil’s Eye from a nearby underground cavern.
Yet trouble abounds as Bernard and Bianca must use their ingenuity and bravery to assist Penny further. When they finally locate her, urging her to escape, a cascade of events leads to Medusa brandishing a shotgun at the captured duo. Time is of the essence as chaos unfolds, and the mice must rely on Evinrude to gather their newfound friends for a daring rescue attempt.
After a series of heart-pounding encounters, ingenuity shines through when Penny manages to procure the diamond while navigating a collapsing cavern, all under the oppressive gaze of Medusa. The climax arrives as Medusa's plan backfires, leaving her agitated as she loses both Penny and her coveted diamond.
In a thrilling turn of events, a band of swamp creatures arrives just in the nick of time, assisting the mice and Penny in making a daring escape. The victory seems bittersweet yet triumphant, as the group narrowly escapes the clutches of danger, utilizing the very swamp juice crafted by Luke to fuel their escape vehicle given the circumstances.
Back at the headquarters of the Rescue Aid Society, the delegates eagerly watch a news report showcasing Penny’s return to the orphanage, finally finding a loving family to adopt her. The scene concludes with an endearing moment as Penny expresses her gratitude and highlights the assistance she received from her unexpected heroes, the mice. Just as the moment of celebration unfolds, Evinrude arrives urgently, signaling another crisis in need of their quick intervention. Without hesitating, Bianca once again expresses her eagerness to dive into the next adventure alongside Bernard.
