The primary theme of Sir Arthur Conan Doyle's story “B. 24” is truth and falsehood, and the title refers to the narrator himself, who is a prisoner with the number “B24.” Let's explore this more.

The narrator is in prison for murder, but he claims that he never killed Lord Mannering, pointing the finger instead at Lady Mannering. The story is told in the form of a letter written to a man the narrator thinks can help him fight the murder conviction. The narrator swears more than once that the story he is telling is true, and he offers a detailed account of what he says really happened on the night of the murder.

The narrator's story stands against the account given by Lady Mannering, who claims to have witnessed the narrator stab Lord Mannering. We readers are left to determine the truth or falsehood of the narrator's story. While the tale sounds good, there is one clue that the narrator may not be as honest as he claims. He says that he was hiding behind a curtain when Lady Mannering stabbed her husband. He heard what was going on, but he did not see it. Yet later he tells the constable that he saw Lady Mannering stab her husband twice with a knife. Perhaps this was just a slip of the tongue under the pressure of the moment, but it does make us wonder a little whether “B24” is truthful or not.