In the enchanting setting of present-day Scotland, an American couple finds themselves in a quaint bar, where they encounter an elderly gentleman. At their request for a unique tale, he recounts the legendary story of the Loch Ness Monster and the origins of the infamous faked photo that sparked interest around the world.
This captivating narrative transports us back to 1942, amidst the turmoil of World War II, where we meet a brave young boy named Angus MacMorrow. He resides in the grand manor of Lord Killin, located along the picturesque shores of Loch Ness, alongside his loving mother, Anne MacMorrow, portrayed by the talented Emily Watson, and his spirited sister, Kirstie (played by Priyanka Xi). Their tranquil life is interrupted when a new handyman, Lewis Mowbray (played by Ben Chaplin), comes to help around the estate. Angus grapples with the absence of his father, Charles (Craig Hall), a sailor in the Royal Navy whose ship met with disaster almost a year ago, leaving Angus struggling with the painful reality of hope fading into loss.
One fateful day, whilst gathering seashells, Angus—despite his fear of the water—feels an inexplicable pull towards the loch, where he uncovers what appears to be an enormous, mysterious egg nestled in the sand. He hastily stashes it in his father’s shed. Upon his return, he is awestruck to witness the egg hatching, revealing a creature he affectionately names 'Crusoe,' after the beloved character from Robinson Crusoe. Eager to keep his discovery a secret, Angus eventually confides in his sister, Kirstie, and the handyman, Lewis, when the creature hilariously manages to invade Kirstie's bathtub. Lewis enlightens Angus by explaining that Crusoe is a rare "Water Horse," a mythical being that can only exist as a single entity at any time, known to lay one egg before its demise.
Soon after, the peaceful existence of the MacMorrow family is disrupted as troops from the 12th Medium Regiment Royal Artillery arrive, commanded by Captain Thomas Hamilton (David Morrissey), a close friend of Lord Killin. The soldiers establish a camp near the loch as a defense strategy against lurking German U-boats. With the presence of soldiers, Lewis and Angus face a dilemma; as Crusoe grows to an enormous size, they realize they must allow him to roam free in the hauntingly beautiful waters of Loch Ness.
Captain Hamilton, however, disapproves of Lewis's influence on Angus. After a humorous debacle involving a ruined dinner due to the escapades of the regiment's dog chasing Crusoe, the captain decides that Lewis should instill some discipline into Angus’s restless spirit. Yet, determined to connect with his majestic friend, Angus manages to slip away from the rigid training to enjoy exhilarating rides on Crusoe’s back, where he gradually overcomes his fears of the deep blue.
Tragedy strikes when Captain Hamilton leads the MacMorrow family to a scenic overlook of Loch Ness for a military demonstration, but Crusoe is nearly injured by an explosive shell. In a moment of sheer courage, Angus dashes to save Crusoe, prompting anger from Hamilton and confusion from his mother, who is unaware of the magical creature’s existence. Over time, bonding over laughter and shared experiences, Lewis cultivates a friendship with Angus and Kirstie, bringing a lightness that their mother appreciates, regardless of Hamilton's strict demeanor.
Amidst the escalating tension, two opportunistic fishermen, having caught a glimpse of Crusoe, scheme to capture the creature on film for fame. Unable to get a real shot amidst the chaos, they settle for fabricating a hoax, which ultimately leads to the iconic "Surgeon's Photo." This faked image generates fervor among the soldiers, prompting nighttime hunts led by Sergeant Walker (Joel Tobeck), as they search the loch under the cover of darkness.
As the adventure unfolds, Angus sneaks out to reconnect with Crusoe, now a magnificent creature, but is met with unintended danger as the soldiers mistakenly perceive it as a threat. In an act of heroism, Crusoe defends Angus, ultimately risking everything for his young companion. Amidst the pandemonium, Angus’s mother arrives just in time to witness the creature and, despite her initial disbelief, begins to accept the fantastical world her son inhabits.
Everything comes to a head when the artillery, mistaking Crusoe for a German vessel, opens fire. In a display of bravery and unity, Angus, Hamilton, Anne, and Lewis band together to protect Crusoe, leading him to safety before he slips into the abyss of the loch, marking a poignant moment of acceptance for both Angus and his mother as she finally contemplates moving forward from past pains.
At dawn, as Angus watches Crusoe's retreat into the vastness of the sea, he finds peace with the truth about his father’s fate. The mystical tale ends with a nostalgic air as Angus, now transformed by his adventures, hints at the legendary creature's legacy. Years pass, and whispers of Crusoe's sightings continue, but Angus remains steadfast in the belief that his enchanting friend has moved on. The story culminates in a bittersweet reunion, as the couple leaves the bar and a distant echo ties them back to the myth, symbolizing that in nature, life finds a way to continue—perhaps reflected in another creature, hidden just beneath the surface.
