Early in this, Andrew Motion's sequel to his novel Silver, itself a follow-on to Robert Louis Stevenson's Treasure Island, the former poet laureate makes a sacrificial execution of Mr Stevenson.

This symbolic and scrawny Scot, who spent much of the first novel in the crow's nest, falls into the hands of Native Americans who do not take kindly to strangers being washed ashore on their land. Shortly, having survived the foundering of the Nightingale, which went down with almost everyone on board, Stevenson is gone, and the protagonists Jim Hawkins and Natty have been taken captive. An eerie emptiness follows the killing of Motion's literary mentor, the reader aware that from this point the chart has been discarded, and we, like his characters, are truly adrift.

Not for Motion a gradual immersion in the world he so vividly created in his pastiche of RLS's original. That first yarn took as its starting point Jim Hawkins's reflection, at the end of Treasure Island, that "The bar silver and the arms still lie, for all that I know, where Flint buried them."

When Motion picks up that tale, the year is 1802 and Hawkins senior is an irascible, sozzled innkeeper near the Thames, and Jim his skivvy of a son. It's no great surprise when, urged on by Natty, daughter of Long John Silver, he steals his father's map of the island, and the pair set sail to find the treasure for themselves. Nor is it hard to predict that their expedition will not be the piece of cake they expect.

The New World takes place two years later, and pitches readers into a maelstrom of misery, with no concession to those unfamiliar with what's gone previously. Nor is that necessary, though as with any sequel the pleasure is heightened by familiarity.

What follows is initially horrifying, the pair thrown into a foetid hut in the natives' camp, awaiting the arrival of the chief, Black Cloud, to decide their fate. In the meantime, they witness the torture and death of their cellmate. In these grisly scenes Motion steps beyond Stevenson's deliberately restricted range, though in the couple's escape, and their audacious theft of Black Cloud's ceremonial necklace, the old master's touch seems to be guiding the page, preposterous though their foolhardy act feels. From this moment the hunt is on, as the young pair first take refuge with a friendly local tribe, and then attempt to reach the east coast to head for home. In their wake, Black Cloud scours the land in search of the thieves and their plunder.

The theme of The New World is greed and guilt, and the corrupting influence of both. Standing as a cipher for the ruination of America by its conquerors, Jim and Natty's story is in part a morality tale, and in part an elegy for a lost civilisation, in whose destruction they unfortunately collude.

The adventurers' journey is at times engrossing, at others a little tedious and even flat. Nor does the emotional angst between Jim and Natty entirely convince. The constant fear of their pursuer, however, is well done, Black Cloud's fury tap-tap-tapping through the pages as if Blind Pew were once more among us.

For a poet, Motion is admirably restrained in his prose. It gallops along unselfconsciously, as would an old storyteller recalling his youth, growing lyrical only on occasion. His description of the serpentine, mud-banked Mississippi is so powerful you can smell it. It is the sea, though, that Motion seems most to love: "Breeze blew off the land to our port side with a sugary fragrance. Our mates hung out all our sails and the Mungo leaped forward as I knew she would. Birds that a moment before had seemed like spirits of the damned now skimmed around us like angels..."

It is a relief, though no great surprise, to report that Stevenson has not been usurped. As if anyone could. Yet while there is much to cavil at in this uneven work, the last chapter, which ends again on a knife-edge, leaves one keen for more, and soon. That is the sort of greed Stevenson would surely have approved of.