Hergé concluded the arc begun in this story with Red Rackham's Treasure, while the series itself became a defining part of the Franco-Belgian comics tradition.

While browsing on the Brussels Voddenmarkt/Marché aux puces at the Vossenplein in the Marollen, Tintin purchases an antique model ship which he intends to give to his friend, Captain Haddock.

[2] The only one of his crew to survive the boarding, Sir Francis killed Red Rackham in single combat and scuttled the Unicorn; he later built three models, which he left to his sons.

A few days later, Tintin is kidnapped and chloroformed by the perpetrators of the shooting: the Bird brothers, two unscrupulous antique dealers who own the third model of the Unicorn.

Tintin escapes from the cellars of the Bird brothers' country estate, Marlinspike Hall, while the Captain arrives with Thomson and Thompson to arrest them.

By combining the three parchments and holding them to light, Tintin and Haddock discover the coordinates (20°37'42.0" N 70°52'15.0" W, 82 km north of the Dominican Republic[3]) of the lost treasure and plan an expedition to find it.

[6] Some Belgians were upset that Hergé was willing to work for a newspaper controlled by the occupying Nazi administration,[7] although he was heavily enticed by the size of Le Soir's readership, which reached 600,000.

[8] Faced with the reality of Nazi oversight, Hergé abandoned the overt political themes that had pervaded much of his earlier work, instead adopting a policy of neutrality.

[9] Without the need to satirise political types, entertainment producer and author Harry Thompson observed that "Hergé was now concentrating more on plot and on developing a new style of character comedy.

[15] However, as Tintin expert Michael Farr related, whereas Cigars of the Pharaoh and The Blue Lotus had been largely "self-sufficient and self-contained", the connection between The Secret of the Unicorn and Red Rackham's Treasure is far closer.

[18] Seeking further accurate depictions of old naval vessels, Hergé consulted his friend Gérard Liger-Belair, who owned a Brussels shop specialising in model ships.

[21] Red Rackham's looks and costumes were also inspired by the character Lerouge, who appears in C. S. Forester's novel, The Captain from Connecticut, and by the 17th-century French buccaneer Daniel Montbars.

During the fighting, the Royal James was set alight and Haddock escaped but had to be rescued from the sea, following which his bravery was recognised by the British monarch, King Charles II.

It was recorded by David Ogg that this captain and his ship had been separated from their squadron whilst out at sea and so docked at Málaga to purchase goods that could be taken back to Britain and sold for a profit.

[37] The Secret of the Unicorn and Red Rackham's Treasure were the first two Adventures of Tintin to be published in standalone English-language translations for the British market, as King Ottokar's Sceptre had previously been serialised in Eagle in 1951.

Thomson and Thompson find their stolen wallets in the thief's alphabetical files before the E section, reflecting their original names Dupont and Dupond.

[41] The Secret of the Unicorn resembled the earlier Adventures of Tintin in its use of style, colour and content, leading Harry Thompson to remark that it "unquestionably" belongs to the 1930s, considering it to be "the last and best of Hergé's detective mysteries".

[42] They went on to state that The Secret of the Unicorn-Red Rackham's Treasure arc represents "a turning point" for the series as it shifts the reader's attention from Tintin to Haddock, who has become "by far, the most interesting character".

[43] Phillipe Goddin commented on the scene in the story in which Haddock relates the life of his ancestor, stating that the reader is "alternately projected into the present and the past with staggering mastery.

[11] He felt that while religious elements had been present in previous stories, they were even stronger in The Secret of the Unicorn and its sequel, something which he attributed to Van Melkebeke's influence.

[45] Biographer Pierre Assouline stated that the story was "clearly influenced ... in spirit if not in detail" by Robert Louis Stevenson's book, Treasure Island in that it "seemed to cater to a need for escapism".

[46] Assouline also expressed the view that the ancestral figure of Sir Francis Haddock reflected Hergé's attempt to incorporate one of his own family secrets, that he had an aristocratic ancestor, into the story.

[48] He also highlighted that the scenes in which Captain Haddock relates the tale of his ancestor carries on the "merging of dreams and reality" that Hergé had "experimented with" in The Crab with the Golden Claws and The Shooting Star.

He stated that in this section "Hergé offers us an embedded story, a kind of interlude in which the artist, setting aside the use value of objects, takes the liberty of giving them mischievous powers, akin to a certain surrealism".

The Secret of the Unicorn was the fourth to be adapted in the second animated series; it was directed by Ray Goossens and written by Greg, a well-known cartoonist who was to become editor-in-chief of Tintin magazine.