NIM LAY ON the floor of the cave with Fred under one arm and Selkie sheltering her back. Through the opening they could see the rain trickle to a stop and the gale gentle to a wind.

They crept out to look, peering out over the Black Rocks.

The sea was still monstrous. The wind had whipped it to a fury, and it wasn’t ready to calm down just because the storm had passed. Towering waves crashed onto the shore; spray foamed white and rainbows fountained into the clearing sky.

It could have been beautiful, if Nim hadn’t known that Jack was on the west side of the island, being tossed even farther from home, and Alex was on the east being thrown towards it.

‘She’ll be smashed on the rocks!’ Nim cried.

She searched the horizon with her spyglass. There was no sign of a sail, but there was a speck that could be a boat with someone inside it.

Nim was still scared, but there wasn’t enough time to think. She crawled back into the cave and tore a sheet from her notebook.

Dear Jack,

I’ve gone to rescue Alex Rover.

Love, Nim

Selkie was waiting; Fred was starting to slide down to the sea.

‘Wait!’ Nim shouted. ‘You can’t swim out there by yourself!’

But Fred wasn’t by himself. Chica had heard Nim’s whistle, a storm-while ago, and had been resting on the seabed as close as she could get to her friends in the cave. Fred scrambled onto her back and hooked his claws to the edge of her shell, staring out over her head.

Nim hugged hard around Selkie’s neck and they slid into the water.

It was hard swimming, even for a brave and determined lion of the sea. The waves slammed against them, so hard and so high that sometimes they threw Selkie backwards and under the water – which is where sea lions like to swim, but not when they’re carrying girls on their backs.

Nim’s hair whipped like long, wet ropes, and she gulped salt water with every breath, but she clung as tight as she ever could.

The seventh wave came; a swamping, dumping wave, stronger than Selkie, forcing her down deep below the water; stronger than Nim, ripping her arms away from Selkie’s neck and pushing her deeper still, so there was nothing but swirling blue and she didn’t know which way was up to the air and which was down to the bottom.

She was whirling … struggling … sinking … but Selkie somersaulted backwards and pushed her up through the water till she was spluttering … coughing … breathing again.

Then the waves weren’t quite so wild; they could ride up them and slide down without going deep under the water. When they were in the valleys they couldn’t see anything but blue, but when they were on the top they could look around. They saw Chica and Fred, but never Alex.

They went on looking. Looking and looking. On and on.

Now Selkie was too tired to leap across the waves and pretend it was fun, but when Nim tried to be helpful and swim, Selkie honked a cross ‘NO!’, so Nim stayed on. She didn’t know if she could have swum in those waves anyway.

She started to worry about what she was going to do if she didn’t find Alex.

She started to worry about how big the waves were where Jack was, and how far away they were taking him.

She started to worry about Chica and Fred.

She started to worry about how she’d know when it was time to turn back – and that was the hardest thought of all, because it was the only one she could do something about.

She was worrying so hard that she nearly fell off when Selkie barked again.

Then Selkie didn’t seem tired and Nim didn’t feel worried; she blew her whistle as hard as she could and they charged across the waves to the small sinking boat.

THE WATER WAS up to Alex’s waist, then her chest, and up to her neck; she was spluttering and ducking, and still struggling with the knots under the water.

She tried to think what a Hero would do now, but all she could think was that a Hero would have known how to untie the knot, and that was no help to her at all. She took a deep breath and wondered if it was the last air she’d taste.

A whistle shrilled – and there was the strangest, most wonderful thing she’d ever seen: a wild-haired girl blowing a shell and riding a sea lion across the waves.

‘Ni —’ Alex shouted, and then her mouth was under water, too.

Nim remembered the scene in Mountain Madness where the Hero was snagged by the rope around his waist as he climbed down a cliff. She grabbed her knife from its pouch and, as Selkie ducked under the water, Nim cut the rope.

Alex bobbed straight up to the surface.

‘Nim Rusoe, I presume?’

The boat gurgled rudely and sank.

They looked at each other and started to giggle, choking with laughter and seawater and Selkie had to bark twice to remind them that treading water with one arm and holding onto a sea lion with the other, in the middle of a stormy ocean, is not the time to giggle. But when Alex heard Selkie bark, she said, ‘So you’re not a Saint Bernard! But you are a saviour!’ and they laughed a bit more.

And then they turned back towards the island.

It should have been easier now they were going the same way as the waves, except that the waves were smashing onto the Black Rocks and they didn’t want to do that, so they decided to curve around the reef and come in on the gentle slopes of Shell Beach. And Selkie couldn’t carry them both.

‘We could take turns swimming and riding,’ said Nim.

But even going with the waves, Nim couldn’t swim as fast as Selkie, so Selkie kept having to turn around, and Alex wasn’t very good at riding, so she fell off every time Selkie turned around.

‘I wish I’d gone to swimming lessons instead of writing stories about fish!’ Alex muttered, and Nim started worrying again. She was getting tired, too, and so the next time Selkie circled back and Alex fell off, they both hung on to Selkie with one arm and swam with the other – but the island didn’t seem to be getting much closer.

Suddenly there was a tickle under Nim’s arm, and a sweet spiny head with a grinning dragon face.

‘Fred!’ she cried – and Alex, who had never seen anything quite as ugly and wonderful as a marine iguana, said, ‘I’m sorry I called you a poodle.’

Following Fred was something that looked like a big bag with something else behind it – and that something began to look like a smug green turtle.

Chica had found one of the coconut rafts.

She moved steadily towards them, pushing the raft, disappearing as each wave rolled over her and popping up when it cleared.

Nothing ever upset Chica.

Selkie kept pushing Alex and Nim towards the island; Chica kept coming closer … and finally they met. Alex slid across from Selkie and climbed onto the coconut raft. She lay down and kicked her legs, but kicking Chica in the head didn’t seem a good way to thank her, so she crouched forward and paddled with her hands. It didn’t make the raft go much faster, but Alex felt better doing something.

Nim got back onto Selkie and Fred climbed onto her shoulders.

A few minutes later they tumbled onto Shell Beach, below where the hut used to be.