Hawker and Jude stared at the photo on the computer screen. They could see water, and something big and grey, half in and half out of the water.

Jude laughed.

"This photo came by email from Edinburgh," said Arla. "Look…" He turned to his computer and opened a file. "It's a night-time photo and not very good."

"It's got arms and a head," said Hawker. "But what is it?"

"What do we do with it when we find it?" asked Jude.

"It kills sheep, cats, and dogs," Arla said. "And perhaps small children next. People in Fort Augustus are screaming at London, and London is screaming at us. Go and find it."

Hawker and Jude waited for a minute, but Arla turned away and began to read his emails.

"Oh, great," said Hawker. "Are we looking for the Loch Ness monster -- old Nessie?"

"Don't laugh," said Arla. "You are looking for a monster."

"I want you to take the next plane to Scotland," said Arla. "Then get up to Loch Ness."

"Kill it," said Arla. "Before it kills you."

"Oh, come on, Arla," Jude said. "You don't mean that."

"Are you telling us everything, Arla?" asked Hawker.

They talked to people in their hotel, and to people in the streets in Fort Augustus. Everybody had a lot to say.

"Of course." Arla put his hands on the table. There were only two fingers on his left hand. "I always tell you everything. You know that."

"Huh!" said Jude.

"How are we going to find this thing?" asked Jude.

They took a plane to Inverness, then drove along the side of Loch Ness to Fort Augustus. It was a grey November day -- grey sky, grey water, grey hills.

Hawker looked across the loch. "It's easy to believe in monsters here," he said. "Loch Ness is about 200 metres deep. The third deepest lake in Europe. Did you know that? A hundred monsters could live down there."

"Talk to people in Fort Augustus first," Hawker said. "Then take it from there."

"It killed Mrs Fraser's dog," said one woman. "She has a house down by the water. The monster came into her garden one night. Her little dog went to look, and the monster killed him -- just like that."

"Mmm. Interesting," said Jude. "But tell us about the Loch Ness monster and your dead sheep."

Dugald's brother, Archie, lived at Invermoriston, about nine kilometres up the loch. He was a big man, with black hair and blue eyes. He was not very friendly at first.

"It wasn't Nessie. Nessie doesn't kill sheep," Archie said. "This… thing… broke their necks."

"Killed six of my brother's sheep," said a man called Dugald. "He's very angry about it."

There's an army laboratory in the hills above the loch. Lots of strange people there. And lots of boats on the loch at night. Coming and going."

"Did you see it?" asked Hawker.

"Right," said Hawker. "Now, we'd like to look at your dead sheep, please. And then we need a boat. Where can we get a fast boat, Archie?"

"Can we meet your brother?" asked Jude. "Ask him some questions?"

"No," said Hawker. "Why do you ask that?""

"Well, I saw something big, but it moved very fast. It was back in the loch in seconds. And it was nearly dark."

"Who are you?" he asked. "Are you army people?""

"What do you want a boat for?"

"We're going to catch this thing," said Jude, "and stop it killing your sheep."

Fifty metres away there were some tall trees by the side of the loch. There were some sheep under the trees -- and something big and dark. Suddenly the sheep began to run.

"Look," whispered Hawker. "Down there, by the trees."

"Start the engine," said Hawker. "Let's get down there."

For the next four days Hawker and Jude went up and down Loch Ness in Archie Campbell's boat. They talked to everybody around the loch. "When you see this monster, please ring us. Any time. Day or night. Our mobile phones are always turned on."

They slept a little in the day, and were out most of the night in the boat, with their guns, cameras, mobile phones, a big flashlight, and litres of hot coffee. Once they thought they saw the monster just north of Invermoriston. Another night a call came from a little place called Foyers. They got there at six in the morning. It was dark and cold, and everything was very quiet. They sat in the boat, looking up and down the loch.

When they got to the trees, they found five sheep with broken necks, but no monster.

The next night they were near Urquhart Castle when they saw two other boats. It was three in the morning.

"Those boats again!" said Hawker. "We heard their engines last night. And the night before. Who are they? Let's go and see."

Quietly, their boat moved nearer. There were four men in each boat -- men in grey, young men with hard faces. They watched Hawker and Jude, and said nothing.

"We're visitors?" said Jude. "Looking for Nessie, of course. What are you doing?"

"It just kills, and goes away again," said Jude. "Horrible."

A tall man in the nearest boat answered. "Army," he said. "Who are you? What are you doing out here at this time of night?"

"Hullo," said Jude, with a friendly smile. "Who are you?"

"There's no monster in this loch," said the tall man. "Go back to your hotel and go to bed." The two army boats moved away, down the loch.

Arla phoned them every morning. "Get on with it," he said. "Before this thing kills someone."

"Nice," said Hawker. "Very friendly."

"You can sleep later," said Arla. "Just catch this thing."

"We can't get near it," Jude told him. "This loch is thirty-six kilometres long and two kilometres across. There are only two of us, and we're not getting any sleep."

"We met some army people last night," Jude said. "What are they doing here? What do they do at their laboratory?"

The phone went quiet. Then, "What laboratory?"

The phone went dead. "Well, well," Jude said to Hawker. "He doesn't know. Or he's not telling."

"Nessie never killed anything in fifteen hundred years," he said. "This is something different, and it's going to start killing people soon. Look, you need some sheep."

The monster came out of the loch and killed sheep nearly every night. Everybody around the loch wanted to help Hawker and Jude, and their mobile phones never stopped ringing. A lot of people thought the monster was Nessie, but Archie knew it wasn't.

"The army laboratory in the hills above Loch Ness."

"Sheep?" said Hawker. "Do you mean sleep?" He was very tired after ten nights on the loch.

"Of course!" said Jude. "Clever Archie. Can you get us some old sheep then? Or perhaps we can use Hawker. He's nearly an old sheep by now."

"Oh, shut up," said Hawker.

They found a good place between Invermoriston and Urquhart Castle with some trees by the water, and took their six old sheep there. For three nights they watched, but no monster came. The men in grey were out every night too. They were never far away from Hawker and Jude, but never came to speak to them.

On the fourth night Hawker and Jude arrived at their sheep place later than usual. It was a warm, rainy night -- soft Scottish rain that never stops.

"Where are the men in grey?" whispered Hawker.

Then everything happened at once. There was a sudden noise of boats' engines behind them, and at the same time something moved in the water in front of them. A head and two long arms came out of the water.

When they turned the engine off, everything was quiet -- just the rain whispering on the loch. Hawker got out of the boat and stood with the flashlight, watching and listening. Jude sat in the boat, with her gun in her hand.

"Don't know," Jude whispered back. "Behind us, I think."

Hawker turned on the flashlight, and at once the monster stood up and ran at Hawker through the water, its long arms going for Hawker's head.

"Light!" shouted Jude. "Quick!"

A minute later, the men in grey arrived in their two boats. They got out and came to look at the dead monster.

"Get down, Hawker!" screamed Jude. In a second Hawker was under the water, and then came the sound of Jude's gun -- once, twice, and then a third time.

"Oh dear," the tall man said. "Poor old Nessie."

When the monster stopped moving, Hawker and Jude went to look at it. It was a greeny-black colour, bigger than a man, and with very long arms and legs. It had a small head on a long neck. It was a strange, horrible thing.

"That", said Hawker, "is not Nessie. So what is it?"

Hawker and Jude stared at him and said nothing. Then Jude took something out of her pocket and held it up, in front of the tall man's eyes.

"Visitors, aren't you?" said the tall man. "Where from?"

He looked at it. "Oh," he said. "I see. EDI. Don't see many of those. OK, what do you want to know?"

"You can make a lot of living things in a laboratory these days," said the tall man. "Genetic engineering is getting very clever. This fighter can run all night, live under water, see in the dark. But it's new. We haven't got it right yet."

"What -- is -- that -- thing?" said Jude.

"But it was alive… a -- a living thing," said Hawker.

"And it's programmed to kill anything, is it?" said Jude.

"No, no," the tall man said quickly. "Not this one. This one just kills small animals with four legs -- sheep, dogs, cats, things like that."

"And where are the other monsters?" asked Hawker. "For killing men, women, and children -- things like that."

"Well," said Jude, "that is nice to know."

The tall man looked unhappy. "It is, or it was, one of the army's new fighters. It got away from our laboratory, and we just couldn't catch it."

The tall man smiled. "Can't answer that, I'm afraid. Talk to your people in Brussels. Well, we must get back. Lots to do, you know. Thanks for your help. Bye."

The men in grey put their dead monster in one of the boats and went away up the loch into the dark and the rain.

"Did the army make any more monsters?" I asked.

"He was so angry!" Hawker said. "He went crazy! He didn't know about the monsters, you see. Or the laboratory. The army didn't tell anyone in Brussels."

"And Arla liked to know everyone's secrets," said Jude.

"And what did Arla say about it all?" I asked.

"Not in Scotland," said Hawker. "The British army closed their laboratory after that. The Americans and the French made some monsters years later. They were smaller, like big cats, but with eight legs."

"Where are they now?" I said. "You don't hear anything about them."

"They're in Antarctica. Or in a place deep under the ground," said Hawker. "Nice and secret. Just waiting for the next war."

"Nessie?" said Jude. She looked me in the eyes and did not smile. "Of course there's a Loch Ness monster. She's a blue-grey colour, has a small head, a very long neck, and is about fifteen metres long… Go and look at all the photos of her on the Internet."

"Of course not," said Jude. "People don't want to hear about monsters."

"And so there isn't a Loch Ness monster, and never was one," I said.