Under Loch and Key

Keyanna "Key" MacKay, a 27-year-old New Yorker, travels to the Scottish Highlands to scatter the ashes of her father, Duncan, who had Alzheimer's disease and died of pneumonia. Key's mother died in childbirth, making Duncan her only family. In his final lucid days, he asked Key to scatter his ashes at Skallangal Cove on Loch Ness, where he claimed a mysterious creature once saved him from drowning. Key has also come to meet her estranged grandparents, Rhona and Finlay MacKay, who cut contact with Duncan after a falling-out decades earlier.

At the cove, Key meets Lachlan Greer, a sharp-tongued Scotsman who pulls her away from the dangerous rocks and calls her "stupid" for ignoring the warning signs. When she reveals she is Rhona's granddaughter, his demeanor shifts to wariness. He drives her to the MacKay farm, where he works as a farmhand. Rhona reluctantly lets Key stay if she earns her keep, while Finlay is warm and welcoming. Key also meets her cousin Brodie MacKay, on sabbatical from the Inverness Historical Society.

Lachlan has spent six months on the farm secretly searching for something to break a curse on his family. His only confidants are Blair and Rory Campbell, twins who co-own the local pub, The Clever Pech, and who witnessed his first transformation as teenagers. Lachlan resents Duncan for abandoning his father, Callum, and has been warned his whole life about a "daughter of MacKay."

One night, Key overhears Rhona telling Finlay that Key is "not really" a MacKay. Devastated, she goes to the cove and scatters Duncan's ashes. Moments later, something massive knocks her into the water. The Loch Ness Monster attacks her, but a second creature fights the first to protect her, sustaining a severe bite wound. Key presses her jacket to the wound on shore, feeling heat surge through her hands and seeing a faint glow before passing out.

She wakes beside a naked Lachlan. He claims he pulled her from the water, but the partially healed bite mark on his shoulder betrays him: He transforms into a beast every night from sundown to sunrise. Key offers to keep his secret, but Lachlan refuses. The next day, Finlay brings Key home, and Rhona opens up. The real break with Duncan came when he announced his wife was pregnant with a girl and declared he would never bring the child to Scotland. Rhona told him not to return, and they never spoke again. She admits that seeing Key fills her with guilt, and Key forgives her.

Key persists until Lachlan shares his family's story. Nearly 800 years ago, his ancestor bargained with a kelpie, a shapeshifting creature from Scottish folklore whose power resided in a magical bridle. According to the Greer family's account, the kelpie betrayed his ancestor after receiving sanctuary, cursing his line so that every Greer son transforms into a beast. The other monster in the loch is Callum, who lost control of the curse when Lachlan was eight and never changed back, becoming permanently trapped in beast form. Lachlan's mother left, and his grandmother raised him. He reveals that the MacKay family descended from the kelpie herself, meaning Key has kelpie blood.

Despite his mistrust of MacKays, Lachlan accepts Key as his ally. Strange things have been happening to her: windows opening at her will, rain stopping at her command. Lachlan suspects dormant kelpie magic is awakening in her. They hike to the ruined Greer castle to search for the bridle. The floor collapses beneath Key, dropping her into a hidden dungeon where the kelpie was once imprisoned, its walls carved with scenes of her captivity. After Lachlan shields her from a falling beam and kisses her, he pulls back, fearing he could lose control. Key refuses to accept this, and they begin a relationship.

As they grow closer, Rhona mentions that Duncan once found an old journal hidden in the barn. Key searches Duncan's childhood bedroom and uses her magic to locate a hollow compartment under the floorboards. Inside is the journal and a letter from Duncan to Rhona, in which he calls himself a coward for leaving Scotland to protect someone. The journal, written by Tavish MacKay, the stable hand who freed the kelpie, rewrites everything Lachlan has been told. His ancestor was not a betrayed benefactor but a brutal tyrant who imprisoned the kelpie Sorcha and exploited her magic after she fulfilled her end of their bargain. The curse was punishment, not treachery.

Key brings the journal to Lachlan, who is devastated to learn the Greers were the villains, not the victims. On the final page, Sorcha writes that she chose to die as a mortal alongside Tavish and left her bridle with him, calling her magic her heart. Her message includes the full text of the curse, containing a second verse Lachlan never shared: The curse can only end with "a daughter of MacKay" who chooses to forgive. Key confronts Lachlan, hurt not by the revelation but by his betrayal of trust. He apologizes, and she leaves to think.

Rereading Duncan's letter, Key realizes her father read the journal, saw the reference to a daughter of MacKay, and fled Scotland to protect his unborn daughter. She also deduces where the bridle has been hidden: Sorcha left it with Tavish, and an ancient leather strap adorns the horse-head tombstone on his grave. Rushing to Lachlan's cottage, she finds it empty with blood on the grass. Her magic leads her to the graveyard, where Lachlan lies unconscious and chained, with Brodie standing over him holding a knife. During renovations of an old Greer building, Brodie found a journal detailing the curse and later witnessed Lachlan's transformation. His claims were dismissed as delusional, costing him his career. Driven by a lifetime of unfavorable comparison to Lachlan by his own father, Brodie wants the bridle's power.

Lachlan breaks free and tackles Brodie, who stabs him in the shoulder. Key retrieves the bridle from Tavish's tombstone and offers it in exchange for peace, but when Brodie advances on Lachlan again, she channels her magic and sets him on fire. Brodie falls, cracks his head on a tombstone, and loses consciousness, but the bridle in his grip burns to nothing. Key heals Lachlan's wound and despairs at having destroyed the bridle. Lachlan tells her the magic is in her, not the object.

Key recalls Sorcha's words: The curse ends when a daughter of MacKay chooses to forgive. As the sun nears the horizon, she and Lachlan declare their love. Key channels energy into him through a kiss and speaks the words: "I forgive you." The sun sets, and Lachlan does not transform. They run to the loch, where Callum emerges on unsteady legs, human for the first time in 26 years, and embraces his son.

Key decides to stay in Scotland and gives Rhona the journal and Duncan's letter. Brodie is transferred to a psychiatric facility after his claims about monsters are dismissed. In an epilogue four years later, Key and Lachlan are married, living on Greer land with their two-year-old daughter, Sorcha, named for the kelpie, who has inherited magical abilities. The extended family gathers for the child's birthday, and Lachlan reflects that the woman he was taught to fear became the one who saved him.