During the German air raids of London in 1940, an elderly and dying Dr John Watson (David Shackleton) watches from his home. He begins to tell a story about an adventure he had with his deceased friend Sherlock Holmes; a tale which they vowed never to tell the public out of personal grief and a belief that the world was not ready to know about it.
Several incidents in 1882 lead to the consulting-detective Sherlock Holmes investigating with aid from his companion, Dr Watson (Gareth David-Lloyd). They investigate a ship carrying gold which was destroyed by a mythical giant octopus on the coast of Newhaven They do not believe the first-hand accounts of the sole survivor of the attack (Neil Williams), but nonetheless investigate. When investigating the remains of the ship on the coast, Holmes is told by Inspector Lestrade (William Huw) that he has recently had contact with his estranged brother Thorpe.
In Whitechapel, a young man is killed by a small Tyrannosaurus rex. Watson does not believe article about the monster until he and Holmes are on a walk in some woods, and the dinosaur appears and chases them. Finding escape, they come across several more clues that make Holmes believe the monsters are artificial, built by a criminal genius. On the case, the dinosaur steals a water pump operating a fountain and much copper wire, raising questions as to what the creature's inventor is trying to achieve. The octopus that destroyed the ship earlier is linked to the dinosaur because they are both similarly "exceptionally improbable". Lestrade is also investigating the crimes and stalks Holmes and Watson on their mission to solve the mysteries. On one of their leads, Lestrade ends up missing. Holmes' astounding logic leads himself and Watson to an old castle in Helmsmouth he and his brother visited as children. They come across another monster, a mechanical man: Spring-Heeled Jack.
The person under the mask is revealed to be Holmes' brother, Thorpe (Dominic Keating) (who was also seen earlier as a patient of Watson's) and Miss Ivory, his accomplice. Thorpe explains that the crimes were committed to make an armory that would keep him alive from the bullet wedged in his spine. All the tools his monsters stole, the gold, the water pump and the copper wire all contributed to the creation of his suit. He plans to destroy London and force Lestrade, who he believes shot him years ago, to take the blame. Holmes tries to attack Thorpe, only to be shot and presumably killed by Miss Ivory.
Miss Ivory is revealed to be one of Thorpe's robotic creations (and his lover), and Thorpe has placed a bomb in her workings that will detonate when she reaches Buckingham Palace, home of Queen Victoria, while Thorpe pilots his most complex invention yet, a fire-breathing dragon in which he pilots and holds Lestrade hostage. Holmes returns, having survived the gunshot as his tobacco box in his pocket stopped the bullet. Watson is sent to stop Miss Ivory from assassinating the Queen, while Holmes pilots another one of Thorpe's inventions, a hot-air balloon driven by helicopter propellers and armed with guns, in an attempt to stop his brother.
Thorpe sets fire to Parliament and Westminster Abbey in his battle with his brother. Ivory is deactivated by Watson moments before the explosion can take place and Thorpe's dragon is sent crashing in the garden outside the Palace. Thorpe crawls from the wreckage and tries to shoot Watson, before he himself is killed by Sherlock Holmes. Lestrade, as usual, takes credit for solving the case. Holmes and Watson vow never to speak of the events again, because of how it personally affected Holmes and his certainty that the world is not ready.
In present time, Watson dies and his nurse visits his grave. She sees Miss Ivory visiting the grave of Thorpe Holmes, questioning her thoughts that the story is false.
- Ben Syder as Sherlock Holmes
- Gareth David-Llyod as Dr John H. Watson
- Dominic Keating as Thorpe Holmes/Spring Heeled Jack
- William Huw as Inspector Lestrade
- Elizabeth Arends as Anedisora Ivory
- David Shackleton as Old Dr Watson