Sherlock Holmes and the Secret Code – aka Dressed to Kill – is the final film in the series, and a very fine film it is too. There's a great cameo by Edmund Breon as 'Stinky' Emery, and the glamorous Patricia Morison plays Mrs Hilda Courtney; a villain with a dab hand at disguises. Courtney leads a criminal gang that's in a race against Holmes to recover the Bank of England's printing plates for £5 notes, hidden somewhere in London by her gaoled accomplice.

Synopsis

The film opens with an inmate of Dartmoor prison making musical boxes, to sell later at Gaylord's auction house. There are three musical boxes that each play a slightly altered version of the same tune, and they're each sold to three separate bidders; a toy shop owner, a Mr Kilgour and the third is sold to Julian Emery. After the sale, a member of a criminal gang, a certain Colonel Cavanaugh, visits the auction house and manages to get the addresses of the people who bought the boxes from the owner.

Julian Emery soon suffers a break-in and tells Sherlock Holmes about it while he's visiting his old school friend, Dr. Watson. Emery's perplexed that the thieves took only the plain wooden musical box, and not any of his more valuable pieces. Holmes realises that the thieves were after the specific box bought at auction, so he listens intently to the tune it plays and knows for sure that this was no ordinary break-in.

That evening, Mrs Courtney makes a late night visit to 'Stinky' Emery in order to charm him into parting with the musical box. Just as she's about to get her prize, her henchman Hamid kills Stinky in a fit of jealousy. Mrs Courtney picks up the musical box and flees to her waiting car. She is justifiably concerned that Holmes is now sure to be on their trail.

In the morning, Holmes and Watson confer with Inspector Hopkins at Stinky's flat, Watson is racked with guilt and Holmes knows that the plain musical box will prove to be missing. The pair next visit Ebenezer Crabtree at the auction rooms, where Crabtree gives them the address of Mr Kilgour and a clue as to the whereabouts of the toy shop owner.

Holmes and Watson make their way to the home of Mr Kilgour and meet the housekeeper on her way out. They soon find the Kilgours' daughter shut in a cupboard, gagged and bound, who tells them that the imposter housekeeper - a disguised Hilda Courtney - has taken the box. Courtney's gang now have two of the boxes and time is running out for Holmes.

Mrs. Courtney and Colonel Cavanaugh go to see the toy shop owner, Evelyn Clifford in the hope of buying the final musical box. Unfortunately for them, Holmes got there first this time, and has left his calling card for Mrs. Courtney. The musical box offers no clues to the location of the printing plates, so Holmes must identify the tune the box plays in order to decipher the code; and so he goes to see an old underground contact, Joe Cisto.

Joe knows every tune that's ever been written, and he quickly identifies the musical box's song as 'The Swagman', after Holmes plays it out on the piano. Holmes and Watson return to Baker St. to find that Courtney's men have been and gone after a good rummage through their rooms for the musical box. Holmes finds an important clue in the form of a discarded cigarette.

The next morning, Watson inadvertently gives Holmes the answer to deciphering the musical code when he mentions the numbering of piano keys. Holmes successfully decodes the message in his musical box, and when combined with the part he memorised from Stinky's box, it reads; “Behind books, third shelf. Secretary, Dr S”. They now have the first and second portions of the message, while the gang has the first and third parts.

Holmes visit's Peterson's Tobacconists to discover the purchaser of the discarded cigarette, and gets Mrs Courtney's address from the helpful proprietor. The discarded cigarette was left deliberately and Holmes walks into a trap. He's led off to a garage where Col. Cavanaugh and Hamid set up a contraption to fill the garage with the poisonous fumes of mono sulphide. Holmes is left to die but manages to escape just it time.

Whilst Holmes is busy in the garage, Mrs Courtney pays Watson a visit. Using a smoke bomb, she manages to get him to reveal the location of the musical box as it's the first thing he grabs when he think's there's a fire. Mrs Courtney takes the box in the confusion, and so she now has all three musical boxes and it's only a matter of time before she retrieves the printing plates.

Holmes gets back safely to Baker St. and as Watson is treating his injuries, he gives Holmes the clue to the missing line of the address in the code. He quotes a line from Dr Samuel Johnson, the 'Dr S' in the message, so Holmes now knows the location of the printing plates; behind some books in the secretary on the third floor of Dr Johnson's house.

Mrs Courtney and her cohorts take part in a guided tour of Dr Johnson's house, where they slip away and find the plates, just where the message said they would be. Their excitement is short-lived however, Holmes is lying in wait for them, pistol at the ready which he uses to stop Hamid throwing a knife at him. The whole gang are led away by the police, who, as usual, had arrived after the event. Holmes hands the printing plates to Insp. Hopkins and the film ends with him giving Watson the credit for solving the case.