It begins in Zennor, the small Cornish town where Dunmore set her first adult novel, Zennor in Darkness, which already has a mermaid legend of its own. Inside Zennor’s church is a small, carved figure supposedly commemorating how a mermaid fell in love with a mortal man and carried him away with her. Sapphy, the narrator, first hears this tale from her father Matthew, who seems to know an uncommon amount about the Mer. He, too, has a fine voice and sings the ancient Scottish lay I wish I was away in Ingo. By the end of the first chapter, he vanishes, and his wife and two children must find a way to continue their lives.

The sorrow of losing a parent is so sensitively described that for the first three chapters you fear that this is going to be one of those children’s books in which magic never really gets going. Be patient: the Mer really exist, as Sapphy discovers when she comes across her brother Conor talking to a girl sitting on the edge of a rock.

Before long she, too, has been seduced into discovering her ability to breathe underwater, thanks to Faro, a boy whose sexiness is only underlined by the possession of “the curving tail of a seal”. He pities her for being “divided”, and persuades her that Conor is in danger. In fact, as we come to realise, Sapphy is far more at risk than her brother, for the Mer blood in her is powerfully attracted to the underwater world — especially once her mother accepts the loss of her husband and starts to go out with a handsome diver, Roger. The electric thrill of swimming with dolphins, of racing along currents and of leaving the world of reason and caution behind are described with glorious intensity:

“We rush onwards, side by side. Sunlight strikes down through the water and we swim in and out of pillars of light and shadow. Below us is white sand, gleaming and glittering. The pull of the tide has made deep ridges in so it looks like ploughed land.

“ ‘Look up,’ says Faro . . . and there’s a brilliant skin of light way up above us, wobbling and shimmering. ‘That’s the surface,’ says Faro, ‘Air.’ ”

Before long, Sapphy is looking “washed out”, refusing food, and it needs the earth magic of Granny Carne and the love of a dog to prevent her following her father. Mermaids have always provided a metaphor for the rich and strange world of the imagination, of a sensuous revelling in the unconscious. Hans Christian Andersen made his Little Mermaid into a cruel parable of the torments of adult sexuality, but women writers from E. Nesbit to Liz Kessler have chosen it as a gentler way of coming to terms with maturity. By returning the Mer to their sinister deep-sea elements, Ingo gets off to a great start, though I suspect it will follow this traditional pattern. Although the first part of this trilogy needs to speed up to hold the attention of children, the lyrical writing and Dunmore’s intense sympathy for all she describes make this a perfect book with which to wind up the summer holidays, or to recollect them.

HarperCollins, £12.99; 272pp
£11.69 (free p&p)
0870 1608080
www.timesonline.co.uk/booksfirst

What's more

THE PIRATES IN THE DEEP GREEN SEA
by Eric Linklater
(Jane Nissen Books)

A must-read for young boys, this splendid underwater thriller has two brothers save the world with the help of magic fish oil. For 6+

THE TAIL OF EMILY WINDSNAP
by Liz Kessler
(Orion, £5.99)

Find out why Emily has been banned from swimming, despite living on a boat. For 7+

THE MERRYMAID OF ZENNOR
by Charles Causley
(Orchard Books, £5.99)

Traditional retelling of the Cornish legend by the poet, with lovely illustrations. For 4+

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