In real life, 221B Baker Street is the home of a bank. When fans send letters to Sherlock Holmes, bank employee Addy Zhuang, known as "Sherlock's Secretary," replies. One day, the bank is robbed, but the thieves only take three letters addressed to Sherlock Holmes.

The police conclude it was all a harmless prank, but Addy suspects something more malicious is happening. With the help of a true crime reporter, Addy and his friends launch their own investigation, which leads to murder and a decades-old cover-up connected to the BBC's "Great Erasure." Can Addy uncover the truth before more people die?