In a vibrant, lush forest, two experienced hunters, including one with an unidentified identity, embark on a quest in pursuit of their elusive prey. However, they soon find themselves wandering into a spellbinding realm teeming with mystical entities, all safeguarded by an enchanting aura. Before leaving, one of the hunters cautions a Unicorn that she might be the very last of her species, a remark that instills a sense of doubt and concern within her. This revelation propels the Unicorn into a daring expedition to uncover the truth about her kind's existence. Along her journey, she encounters a wise and quirky talking butterfly that shares ominous tales about a formidable entity known as the Red Bull, whose dark influence has driven her kin away from their ancestral home.
Undaunted by the looming dangers, the Unicorn is fueled by an unquenchable desire to reunite with her fellow creatures. Her adventure takes a sudden twist when she falls victim to the clever and magical allure of Mommy Fortuna, the mysterious leader of a traveling carnival. Fortuna has a knack for using her spells to morph everyday animals into extraordinary beings. Within Fortuna's carnival, the Unicorn discovers that she is the sole genuine legendary creature amid a collection of enchanted animals, with the exception of the enigmatic harpy, Celaeno. Fortunately, Schmendrick, a charming magician who is entwined with the carnival, recognizes the Unicorn's true identity and aids her in breaking free from Fortuna’s ensnaring spell.
Now liberated, the Unicorn and Schmendrick set out on a mission to free the other enchanted beings, including Celaeno, who ultimately seeks justice against Mommy Fortuna for her deceitful magic. As their quest continues, the duo approaches the ominous fortress of King Haggard, a merciless and cunning ruler shrouded in an ominous atmosphere. However, the journey takes a harrowing turn when Schmendrick falls prey to a gang of bandits, prompting the Unicorn to leap into action to rescue him. In her endeavors, she crosses paths with Molly Grue, the bandit leader's mistress, and together they journey to Hagsgate, the first settlement that fell under Haggard's tyrannical rule.
In Hagsgate, a villager named Drinn recounts a chilling curse that binds their community to the whims of King Haggard's castle, asserting that their fortunes will remain tied to it until a courageous individual rises to confront the ruler. Drinn reveals that he had discovered a forsaken infant in the market one fateful night—the very child that King Haggard adopted, thus setting in motion a series of events that would ultimately change their fates forever. As they draw nearer to Haggard's fortress, the trio—Molly, Schmendrick, and the Unicorn—are suddenly confronted by the relentless Red Bull. In a well-meaning yet misguided attempt to assist his friend, Schmendrick accidentally transforms the majestic Unicorn into a mere mortal woman, Lady Amalthea. The abrupt shift in reality causes the Red Bull to abandon its chase, leaving both stunned and bewildered.
Now grappling with mortality's fragile nature, the former Unicorn struggles to adapt, while Schmendrick reveals that his own immortality prevents him from executing true magic unless he becomes mortal himself. Encouraged by Schmendrick to persevere, Amalthea disguises herself and follows him into Haggard's castle as a servant. Navigating the perilous corridors, Schmendrick introduces Lady Amalthea as "Her Majesty" to the unsuspecting Haggard. As fate would have it, Prince Lír becomes deeply infatuated with Amalthea, entirely oblivious to her unicorn heritage. Haggard confesses to Amalthea that his happiness is bound to the presence of unicorns, which he has imprisoned in the sea for his own selfish gain.
As Amalthea delves deeper into the mysteries surrounding Haggard's reign, she faces veiled threats and accusations. With cryptic guidance from a clever feline friend, Molly, Schmendrick, and Amalthea discover the lair of the Red Bull, joined by Lír. When the beast attacks, Schmendrick makes a desperate gamble, reverting Amalthea back to her Unicorn form while sacrificing his own immortality to do so. Armed with a mix of vengeance, grief, and resolve, the Unicorn channels her powers to drive the Red Bull into the depths of the sea, liberating her fellow unicorns. As Haggard's castle crumbles into the abyss, the liberated unicorns return triumphantly to their forest haven.
In the wake of their hard-won victory, the Unicorn uses the healing power of her horn to tend to Lír’s mortal wounds. With a newfound sense of duty, Lír sets out to rebuild the devastated Hagsgate, leaving Schmendrick and Molly to wander the outskirts of his domain. The Unicorn, now imbued with wisdom from her arduous journey, shares a heartfelt moment with Schmendrick before heading back to her forest, forever altered by her experiences of love and loss.
As Schmendrick and Molly proceed on their travels, they encounter a princess in distress, and Schmendrick advises her to seek out Lír, now celebrated as a hero. The duo caps off their adventure with a melodious duet, blending their voices of longing and devotion, harmonizing across the sprawling landscape.
