Data and Geordi play Sherlock Holmes and Dr. Watson on the holodeck. The computer creates a powerful foe, Dr. Moriarty, who kidnaps Dr. Pulaski and learns to control the ship.

Diana Muldaur (Dr. Katherine Pulaski)
Daniel Davis (Dr. James Moriarty)
Alan Shearman (Inspector Lestrade)

Biff Manard (Ruffian)
Diz White (Prostitute)
Anne Elizabeth Ramsey (Ensign Clancy)
Richard Merson (Pie Man)

• Professor Moriarty returns to the holodeck in sixth season's "Ship in a Bottle".
  
• This is the only episode Data plays Sherlock Holmes, due to a lawsuit from Sir Arthur Conan Doyle's estate.