Wallace Collection calls upon Sherlock Holmes requesting the Great Detective's assistance. Over the past few weeks, someone has been paying nocturnal visits to the gallery and apparently spending a great deal of time in one of the rooms devoted to medieval arms and armour. Although nothing appears to have been taken, the curator wants to know who is visiting after hours and why.

When Holmes inspects the room, he makes a startling discovery that sets in motion a chain of events which ends up pitting Holmes against a modern day Templar. As he follows the clues, he finds himself exploring an underground storeroom that dates from the Middle Ages and traveling to Cornwall to examine the vestiges of buildings erected by the Templars centuries earlier.

All the while, he is being shadowed by an adversary who seems at least equal to Holmes - if not one step ahead. It's a cat-and-mouse game that can have only one winner; unfortunately, no one - not even Holmes - knows what the prize is.