Exactly which countries are involved is somewhat distorted in translation. Up to now it has been natural to assume that Tintin remains rooted in Brussels, having used a train to get through continental Europe and a ferry to reach the United Kingdom. But here a lot has been Anglicised in translation such that the story begins on Old Street, the currency used is (pre-decimal) pounds sterling and, most bluntly, the address of Marlinspike Hall is given as the fictional Marlinshire in England. Captain Haddock's ancestor, Sir Francis Haddock, is also anglicised, now serving Charles II and the Unicorn flies the Union Jack (in its old Anglo-Scottish form), though looking closely at the cover shows that there the ship retains the original French flag and the sailors' uniforms retain a French feel.
It's easy to understand why this was done, as this was one of the earliest Adventures to be translated into English, though it's not immediately clear whether the changes were made for the unsuccessful 1952 edition or the later 1959 edition which had a new translation. Either way this was still an early foray into the British market for Tintin and indeed for book versions of comics as a whole so it's understandable that the publishers were nervous and would do whatever they could to make the character seem closer to home for the readers. But whilst Tintin may not be as aggressively one nationality as some fictional characters, he is still nevertheless a Belgian character created by a Belgian creator and operating out of Brussels. Relocating him to London feels wrong. It gets odder still with Marlinspike Hall being out in the countryside yet close enough for Snowy to follow the van carrying Tintin to there. That's possible in some cities but Tintin seems to live close to the centre of his and London is way too big for such a feat. It's less of a problem with Sir Francis Haddock as he is a one-off character, sailors often have descendants in other countries and Captain Haddock's name (which is unchanged from French and indeed of the six main characters his is also the name least changed for other translations) already implies Tintin's friend he has at least British ancestry.
Setting the adventure at home, bar the extended flashback, makes for a very different approach from before. There's no great exploration of other countries or thinly disguised political analogies. Instead we get a multi-part mystery as Tintin buys an old ship model in a market, only to immediately discover it's in great demand and he steadily unfolds the reasons why. Meanwhile Thompson and Thomson are trying to stop a pickpocket, but keep having their own wallets stolen instead. And Captain Haddock is enjoying the glory of his family history.
The highlight, which also forms the basis of the cover, is the extended scene where Captain Haddock relates the tail of his ancestor, complete with hat and cutlass as he acts out some moments. Both Haddocks are drawn identical, bar the length of their hair, in a rare drawn example of the "identical ancestor" convention more often seen in live-action film and television. The result allows for some seamless transitions between the Captain's flat and the deck of Sir Francis's ship as the latter faced an attack by pirates. The method also allows Hergé to sidestep having to draw the actual moments where Sir Francis kills first the pirate first mate and then later Red Rackham in sword-fights. What could have been a simple account told over a couple of pages to establish the history of the Unicorn is instead a fourteen-page scene but it never once feels like padding and instead it really brings the backstory to life.
The present-day part of the story is primarily a search for three parts of a treasure map, or rather the co-ordinates for one, with Tintin trying to work out what's going on. A complication comes from the ongoing wallet stealing around the city which provides not only amusement with the Thom(p)sons but also real complications in the search. There's a small error in the plot in that towards the end of the story the Bird brothers talk of having stolen the third parchment from Tintin's room but this isn't what happened - instead the parchment had rolled under a chest of draws and is only later stolen when Tintin's wallet disappears. It's an odd moment as ultimately all three parchments are finally recovered at about the same time but it suggests some confusion of the part of Hergé as to just how many pickpockets were operating and perhaps originally the kleptomaniac Aristides Silk was going to be part of the wider plot.
The actual villains of the piece, the Bird brothers, are rather dull villains who seem to have drifted into criminality as part of their antiques business. One of them is also caught in a very anti-climactic way as Tintin simply issues a description to the police and two days later he's picked up at the border. But the showdown introduces one of the best-known locations in the Tintin stories - Marlinspike Hall. A country house with surrounding estate recently purchased by the Bird brothers, it makes for a strong setting as Tintin works out a way to escape first from the cellar and then through the grounds. Also introduced for the first time is Nestor, the Hall's loyal butler who is exonerated for his actions by Tintin.
Being the first instalment of a longer tale it's inevitable that the story devotes a lot of attention to setting up the back story and only has a resolution in revealing the secret itself rather than what it leads to. But it's a good strong adventure that takes a limited, localised setting and turns it to advantage, offering a story with a strong mix of mystery, adventure, action and comedy that all combine well to advance the plot forward.