Reading The Coral Island after a 40-year gap, I found I knew whole passages almost by heart. Had I read it that often? Ballantyne's stories, which began with his own adventures as an employee of the Hudson's Bay Company, successfully nourished generations of potential colonials, and the imperial message is plain. The depictions of the savages, who respond gratefully to Jack's civilising influence, hardened cannibals though they are, are highly suspect reading in the 21st century. The ultimate arrival of the missionary is not only perfunctory in terms of plot – clearly Ballantyne had had enough of his heroic trio at this point – but also ideologically disastrous. And yet the novel remains not only a gripping yarn, but is more ambiguous than I remembered.

Ballantyne was a master of plot. He borrowed unashamedly from Defoe and Wyss, but made their material his own. The discovery of the skeleton in the hut, the escape from the pirates by diving into the underwater cave and the shipwreck suspend disbelief, though details such as the penguins on this tropical island probably offend the more literal-minded.

At one point Peterkin, the junior member of the trio, addresses the cat: 'I love you because I've got to take care of you, and to look after you, and to think about you, and to see that you don't die.' Peterkin has real feelings about his castaway state, and a sense of irony which allows him, and the reader, to discern more than one meaning at a time. The death of the cat, at the hands of the pirates, is the most upsetting moment in this novel full of human killings, because of Peterkin. By the end of the book he has grown up more than the two stuffed shirts ever will.

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