Sherlock Holmes, Dr. Watson, Irene Adler, and Inspector Lestrade must solve thirty of the most difficult and intriguing cases to ever test their abilities. Each sleuth thinks he or she is the most capable of finding the solutions, so they've decided to turn each mystery into a friendly competition of their skills.

The famous four meet at 221 B Baker Street, address of Holmes and Watson. From there, they travel through the streets and alleys of London, gathering clues and trying to outwit each other. The investigators use secret tunnels to gain quick access to other parts of the city. They acquire badges from Scotland Yard to lock the doors of buildings where valuable evidence is contained. Of course, locked-out sleuths can find keys at the Locksmith.

The detectives are armed with secret codes, allowing them to receive exclusive information from fifteen locations around the city. While working to solve the crime, each sleuth is trying to break the others' codes in order to have access to more clues with less work. They can also change their own codes to throw the others off the track.

The winner is the one who solves the mystery, returns to 221 B Baker Street with a badge, and announces the correct solution.