The door was here all along.

Nobody was standing in this room on the last night, which is exactly why the fifty-first case ends in it. You worked that out, so the rest is a formality: prove you have the book, and the whole proof comes to you.

Where the code is

Turn to the last page of The Halmerton Case Files. Under the final line there is a short code in capitals. Type it exactly as printed — spaces and dashes do not matter, and neither does the case.

Not got the book yet? The fifty-first case is the one thing that is not in the sample — but the sample has four complete cases and their solutions, and it costs you nothing. Take the sample instead.