On Dartmoor, twenty years after young Henry Knight, Russell Tovey, witnessed his father torn apart by a monstrous hound near Dewer's Hollow, the same moor returns to threaten a new generation. Henry travels back to the place on a therapist’s advice and brings with him a chilling clue: the footprints belong to something huge, and he suspects the nearby Baskerville government research station is breeding mutant animals. This haunting rumor pulls the famed sleuthing duo into a case that blends folklore with cold science, and it all begins with the pair hearing Henry’s memory and fear in a way only a master observer can parse.
Sherlock Holmes, [Benedict Cumberbatch], and Dr. John Watson, [Martin Freeman], arrive on the misty moor and quickly realize that the local legend may be more than a story. Henry’s insistence on the term “hound” rather than a simple dog intrigues Holmess' keen mind, and the trio heads toward Baskerville, a facility tied to secretive government work. The visit is complicated when Mycroft Holmes, [Mark Gatiss], uses a security pass that triggers an alert, though Dr. Bob Frankland, [Clive Mantle], vouches for Sherlock’s identity, recognizing him as someone who has crossed paths with danger before. The atmosphere grows tenser as Major Barrymore, [Simon Paisley Day], questions the group, while Frankland’s quiet concern for Henry underscores the human stakes behind the scientific experiments.
As Henry shares dream fragments—words like “liberty” and “in”—Sherlock, John, and Henry drift deeper into the mystery. They venture to the hollow at night, hoping a real creature will reveal itself. On the moor, local guide Fletcher, [Stephen Wight], monetizes Dartmoor’s legend with tourist walks and even produces a plaster paw print that would be massive for a dog—far bigger than any ordinary animal. The trio then revisits Dewer’s Hollow, where Sherlock thinks he glimpsed something, but Henry’s certainty that the hound is real makes Sherlock doubt his own doubt. A moment in an inn adds to the tension: a Morse-coded signal with torches and lights proves to be an old couple in a car, not a call from the wind, scaring Sherlock and John but also underscoring how fear can masquerade as truth.
The next morning, Sherlock requests unrestricted access to Baskerville’s lower levels, hoping to uncover the truth behind Henry’s memory. John finds himself trapped, hears growling, and narrowly escapes with Sherlock’s help, a scene that confirms the danger of what might be inside the facility. To crack the case, Sherlock enlists Dr. Stapleton, [Amelia Bullmore], who has her own complicated history with the Baskerville project and a prior case involving her son and a glowing rabbit. Her testimony, coupled with Sherlock’s deductive thrust, points toward secret genetic experiments and a strategic deception that could turn memory itself into a weapon.
A critical breakthrough comes when Sherlock theorizes that the “hound” might be an acronym rather than a creature. The case intersects with DI Lestrade, [Rupert Graves], sent by Mycroft to shadow Sherlock, and they press the innkeepers about a past order for meat that clashes with the vegetarian restaurant’s image. The truth narrows as they discover a dog on the moor used to lure tourists, a cover that satisfies some, but not Sherlock, who remains convinced the creature is monstrous.
With Mycroft arranging access to the Baskerville facility, the group delves into the lower laboratories. John’s solo foray traps him in a cage where he hears the ominous growl, and Sherlock’s rescue is swift and decisive. The investigation grows darker as Sherlock teams with Dr. Stapleton to reveal a secret: a covert genetic program tied to a weaponized chemical that can trigger violent hallucinations. The mind becomes the battlefield, and Sherlock retreats into a memory-based “mind palace” to connect disparate clues. Liberty, Indiana, becomes a key linguistic clue, and the letters linked to “LIBERTY” and “IN” guide them to a project codenamed HOUND, a CIA-driven plan designed to sow fear and confusion as an anti personnel measure.
The plot thickens as Sherlock uncovers that Dr. Frankland, who began his career with HOUND, has continued to pursue its military possibilities. Henry’s safety becomes a political weapon as John receives an urgent warning from Mortimer, [Sasha Behar], Henry’s therapist, and the trio races back to the hollow. There, the terrifying truth unfolds: the gas used to drive hallucinations was the same agent that silenced Henry's father and that Frankland orchestrated the chain of events to cover up his experiments. The fog in the hollow is not a natural phenomenon but a chemical weapon released by pressure pads, and it shapes what the witnesses perceive.
As the truth comes out, Henry calms as the cause of the fear is exposed, but a final burst of conflict erupts. Sherlock realizes that the drug had leaked into John’s lab through faulty pipes, which meant Sherlock had staged a test to see if his theory could hold under real pressure. In a brutal confrontation, Henry’s father had died as a consequence of Frankland’s machinations, not from a straightforward attack, and Frankland’s attempt to cover his tracks leads to a fatal explosion in Baskerville’s minefield.
In the closing moments, the case closes with a chilling tease: Mycroft oversees the release of Moriarty, [Andrew Scott], from a holding cell, and the walls around him seem to spell out “Sherlock.” The specter of Moriarty lingers as Holmes and Watson prepare for whatever comes next, and the fragile balance between truth and memory remains a central theme. The investigation highlights how fear, manipulation, and the reach of powerful institutions can twist even the most rational minds, turning legends into dangerous realities and leaving a mark on everyone involved.
> "I saw the footprints of a gigantic hound."
