After Faulks-does-Fleming, Horowitz-does-Holmes, and PD James-whodunnits-Jane Austen, now Andrew Motion does Robert Louis Stevenson. For those who believe that sequel writing is nothing more than a sweet trade indulged in by drivel-swigging bilge-rats, it might be worth recalling Stevenson's own defence of his work: "To those who ask me  to do nothing but refined, high-toned, bejay-bedamn masterpieces, I will offer the following bargain: I agree to their proposal if they give me £1,000." Treasure Island was all about the pursuit of riches: Silver discovers that treasure still lies buried on the island.

Stevenson, with an eye always fixed firmly on the main chance and the far horizon, left his own novel half-anchored, well-vittled and more than ready to set sail for a sequel, with Jim Hawkins at the end reminding the reader: "The bar silver and the arms still lie, for all that I know, where Flint buried them", and three troublesome sailors marooned and awaiting rescue. Motion's Silver begins with Hawkins long since retired from adventuring and having become an innkeeper. His son, young Jim Hawkins, teams up with Nat, daughter of Long John Silver, to finish off the business their fathers left undone. For all it's about breaking away and seeking new territory, Motion's is also a novel about returning home and fulfilling promises.

The book shares many of the quirks of its predecessor. Episodic, scrappy  Stevenson wrote the first 15 chapters in 15 days, clearly patching it together from beachcombed bits and bobs of Ballantyne and Robinson Crusoe  Treasure Island is a troubling example of a novel that far exceeds the sum of its parts; like all great books, it is mysteriously self-excelling. Stevenson's indomitable wife Fanny initially wasn't keen, though she came round in the end, when the royalties came rolling in (she wasn't daft). "I liked the beginning, but after that the life seemed to go out of it and it became tedious."

Reading Silver, one occasionally feels like an unjust Fanny. The book begins magnificently, with Jim and Nat meeting and preparing for their journey together, and Long John Silver making a spine-tingling guest appearance, but there are undoubtedly passages of tedium, most notably when the narrative becomes self-conscious and sugary ("Every individual has a part to play in the creation of their society and performs it by instinct"). Fortunately, as with Stevenson, the inevitable momentum of the journey carries the reader out of the doldrums, and one eventually comes to admire Motion's "prodigious concentration of watery energy". The prose, if not exactly thrilling, is fluid, as in the description of Jim and Nat's ship, the Silver Nightingale  "a Baltimore clipper, some hundred feet long, with two masts (both distinctly raked), a bowsprit made like a dagger to cut the waves, a flush deck, a beam rather great for her length, but the most graceful and easy lines"  and the description of Long John Silver's home, the Spyglass. "For rather than being made of bricks and mortar, the walls were comprised of planks, spars, branches, roots, pieces of barrel and every other sort of wooden material the river happened to have carried within reach."

Motion's characters also amount to more than themselves alone. If an adventure story is often a tale of growing to maturity through self-knowledge, then Motion's Jim Hawkins remains a peculiarly passive kind of adventurer: his self-awakening, such as it is, occurs in relation to his feelings for Nat, rather than through the completion of a quest. One almost wonders if what we have here is a romance masquerading as an adventure. This might explain also the super-sweet-natured camaraderie of the crew, which includes a Mr Tickle and also, amusingly, a Mr Stevenson, "a Scotsman and a wisp of a fellow", who keeps lookout in the crow's-nest. The most motley among them is Jordan Hands, nephew of Treasure Island's Israel Hands  who was killed by Jim Hawkins Sr  and around whom and through whom a little squall of trouble brews.

One might quarrel further with Motion  there is perhaps a lack of psychological plausibility in Jim's departure, and some rather earnest episodes on the island involving slaves being freed from subjugation  but there is no denying that Silver is a deeply pleasing and convivial book. After years of dutiful service in his role as poet laureate, Motion seems to have entered into a new world of imaginative play and jouissance. As with Treasure Island, Silver is left open to the possibility of its own sequel, and surely no one would wish Motion to swallow the anchor. All aboard! All aboard! All aboard that's coming aboard

Andrew Motion has form with literary re-imaginings of famous stories. In Wainright the Poisoner (2000) he presented the fictional autobiography of Thomas Wainright, Romantic artist and career criminal. His 2004 novella, The Invention of Dr Cake, imagined an alternate ending to Keats's life, in which the poet survives tuberculosis and returns from Rome to become an English country doctor. Now in Silver: Return to Treasure Island he has written a sequel to a fictional narrative, taking up the original story several decades after Jim Hawkins returns to England laden with treasure, and Long John Silver disappears into the wilds of Spanish America.

Its narrator is Hawkins's son, another Jim, whose isolated life at his father's inn is interrupted by the arrival of Natty, daughter of Long John Silver. She persuades Jim to return to Treasure Island with her in search of the silver left behind by Hawkins and his crew, and they set sail in the Nightingale, retracing the journey made by their fathers years before.

Treasure Island, however, is no longer the barely inhabited place it once was. The pirates marooned at the end of Stevenson's story have become depraved creatures who are subjecting the passengers of a wrecked slave ship to unimaginable horror. The swashbuckling violence of the original gives way to real brutality as the crimes of the pirates are revealed and as the crew of the Nightingale attempt to rescue the slaves. Silver becomes a secondary consideration in this incarnation of Treasure Island, and treasure hunting a greedy concern in the face of so much suffering.

There is, however, plenty to keep literary treasure hunters happy. Silver is stuffed with the black spots, treasure maps and sailor's chests that animate its source text, and is an impressive work of ventriloquism as Motion mimics Stevenson's dialogue and tricks of narrative foreshadowing. Stevenson himself makes an appearance among the crew of the Nightingale: appropriately he is the Watch, the eyes and ears of the ship. There is even a nod to Motion's previous work, when he sends young Jim to Keats's school at Enfield. The island, meanwhile, comes to life through vivid evocations of its landscape and its creatures: huge squirrels, terrifying birds and slumbering sea-lions. Motion doesn't quite succeed in capturing the adventurous spirit of Treasure Island but instead does something else entirely. He reinvents Stevenson's world to reveal its dark underside, illuminating both its mysterious beauty and its grim immorality. Silver provides a fitting sequel to Treasure Island, certainly, but also stands in its own right as a companion volume to a literary classic.