Pursuit to Algiers is set upon the high seas, with Sherlock Holmes and Dr. Watson escorting the King of Rovenia to his homeland, wherever that might be. This instalment of the series sees the famous duo accompanying a VIP, so no real mystery to uncover but there's plenty of assassination plots to foil, and a jewel robbery to occupy our protagonists, and a spot of singing too. The film has some great scenes and some very enjoyable minor characters, especially Agatha Dunham and Mirko.

Synopsis

Sherlock Holmes and Dr. Watson are strolling along the foggy London streets when they're approached by a man and inveigled into eating at The Soho Oyster House, where Holmes deduces from various cues and clues that they need to make their way to No. 26 Fish Bone Alley at 8 o'clock that evening. When they arrive, they meet various ministers from Rovenia. Holmes soon agrees to escort Prince Nikolas, heir to the Rovenian kingdom, on a passenger ship from England to Algiers. The Prince is the target of numerous assassins attempting to seize control of the country.

Holmes and the Prince take a plane to Rovenia whilst Watson goes aboard the S.S. Friesland as a decoy. At sea, Watson enjoys himself as he mingles with the other passengers and he soon befriends Sheila Woodbury, a singer from New York. After dinner, Watson reads a news bulletin detailing the shooting down of Holmes' plane, and as he stares out to sea in shock and disbelief, the steward asks him to attend a sick passenger.

The sick passenger turns out to be the prince, accompanied by Holmes, much to the relief of Watson. Holmes was suspicious of the plane flight and so, along with the prince, he hid aboard the S.S. Friesland instead. Prince Nikolas is now to pose as Watson's nephew while on board, the new plan is to sail to Algiers and hand the prince over to other trustworthy agents to complete his journey to Rovenia. Later that evening, Sheila Woodbury is singing at the piano, much to Watson's delight but as soon as Holmes comes in and she's introduced to him, she leaves abruptly.

The next morning, Sheila and Nikolas take a romantic stroll on deck while Watson keeps and eye on them. Watson quickly looses them in the thick fog and once again, when Ms Woodbury meets Holmes, she hurriedly leaves. Agatha Dunham takes charge of Watson's search for Nikolas and soon leads him to “the dear boy”. That evening, the ship unexpectedly docks at Lisbon and three new passengers come aboard, and for the third time, Holmes causes Sheila Woodbury to run off at the sight of him. One of the new passengers recognises Holmes which immediately raises his suspicions, so he moves Nikolas into Watson's cabin as his own has a porthole that could be utilised by an assassin.

Early the next day, the steward serves them all tea in Holmes' cabin whilst Nikolas has coffee, which Watson luckily spots to be curdling the milk. The coffee is poisoned, another attempt on the prince's life has been made. After breakfast, Holmes and Watson play a friendly game of shuffleboard with the trio of passengers that boarded at Lisbon. Their leader Gregor verbally jousts with Holmes and speaks in coded language about killing the prince. Gregor, Mirko and Gubec plan to kidnap the prince and that evening Mirko, a master knife thrower, tries to kill Holmes but is stopped by his ingenious use of a porthole cover.

The next day, Holmes prompts Watson to ask Ms Woodbury to play Flow Gently Sweet Afton on the piano and when Watson looks in her music case, she stops him and runs out onto the deck. Holmes goes outside and corners her, he persuades her to let him look in her music case, wherein he finds some secreted stolen jewels; the Duchess of Brookdale's emeralds. They were planted in her music case by her manager so Holmes takes them so he can return them with no fuss.

On the last evening of the journey, Agatha Dunham organises a party for the passengers and while she sets it up, we're treated to Watson singing The Bonnie Banks o' Loch Lomond. The trio of assassins take advantage of the opportunity that the party presents - the guests all have a cracker at each placing, so Gregor substitutes the prince's party cracker for a bomb. As Watson is regaling the party guests with the tale of The Giant Rat of Sumatra, Holmes foils the plot when he picks up the fake cracker and notices the weight difference, so he places a real cracker in its place and puts the bomb in his pocket, which he disposes of over the side of the ship whilst chatting to the would-be assassins.

As they reach the safety of Algiers, Holmes sends Watson to fetch the prince's entourage by dinghy, and so Gregor seizes his chance. He impersonates Watson and the prince lets him and his accomplices into the room. They kidnap the prince and leave Holmes bound and gagged on the bed. When Watson and company arrive, they find Holmes trussed up in the cabin but alive and well. Holmes reveals that the prince was a decoy; he was posing as a prince who was posing as Watson's nephew. The real prince had been posing as their Steward, hidden in plain sight the whole time.

The film ends with Holmes and Watson chatting underneath the porthole in their cabin, with Holmes advising Watson to never even think about becoming an actor…