'Circle,' shouted Anawak, 'and keep it tight. Lucy will come up pretty much where she went down.'

The DHC-2 banked sharply. Suddenly the sea rose towards them. Danny was balanced like a monkey in the linkage, one hand on the door frame, the other on the crossbow, ready to shoot. In the water below, the silhouette of a whale emerged through the waves. Then a grey shiny hump broke the surface.

'Yee-hah!' screeched Danny.

'Leon!' Ford was on the radio. 'It's the wrong whale. Lucy's ahead of us, starboard side.'

'Hell,' muttered Anawak. He'd miscalculated. Evidently Lucy was determined not to play the game. 'Don't shoot, Danny.'

The plane stopped circling and sank even lower. The waves rushed beneath them. They were approaching aft of the tug. For a moment it looked as though they were going to fly straight into the Whistler's bridge, but the pilot adjusted their course and they passed to the side of the hulking vessel. A little way ahead Lucy dived again, showing her flukes. This time Anawak recognised her from the distinctive grooves in her tail. 'Slow down,' he said.

The pilot reduced speed, but they were still travelling too fast. We should have taken a chopper, thought Anawak. Now they were going to overshoot their target. They'd have to wheel round, in the hope that the whale hadn't vanished again.

But Lucy's enormous body glistened in the sunlight.

'Turn round, then down.'

The pilot nodded. 'Just don't throw up on me,' he said.

The plane tilted abruptly. A vertical wall of water sparkled through the open door, terrifyingly close. Delaware screamed, but Danny cheered and waved his crossbow.

Anawak saw everything in slow motion. He'd never imagined you could turn a plane like a pair of compasses, with the wing tip acting as the point. The plane moved in a perfect semi-circle, tilted again without warning, and then they were upright.

Engine roaring, they headed for the whale, towards the oncoming Whistler.

FORD WATCHED with bated breath as the plane returned from its daredevil manoeuvre. Its skids were almost touching the water. He vaguely remembered that one of Tofino Air's pilots used to fly for the Canadian Air Force. Now he knew which one.

The cylindrical case of the URA was dangling from the Whistler's stern crane on the other side of the railings, ready to be released as soon as the marksman fired the tag. The whale's grey back was clearly visible. The animal and the plane sped towards each other. Ford spotted Danny crouching under the wing and prayed that a single shot would do the job.

LUCY'S HUMP surged through the waves.

Danny raised the crossbow, closed one eye, and bent his finger. Concentrating hard, he pulled the trigger. He was the only one to hear the soft hiss as the tagged dart left the bow at 250 kilometres per hour and whizzed past his ear. A fraction of a second later the metal barb penetrated the blubber and embedded itself in the whale. Lucy arched her back and dived.

'We got it!' Anawak hollered into the radio.

FORD GAVE THE SIGN. The crane released the robot, which splashed into the waves. As it made contact with water, a sensor was triggered and the electrical motor leaped into action. Plunging deeper, it homed in on the disappearing whale. Seconds later, it was out of sight.

Ford punched the air. 'Yes!'

The DHC-2 thundered above the Whistler. Through the doorway Danny raised the crossbow and whooped triumphantly.

'WE DID IT!'

'Nice work!'

'One shot and – Christ, did you see it? Unbelievable!'

Inside the plane everyone was talking at once. Danny started to pull himself into the cabin and Anawak stretched out a hand to help him. Then something loomed out of the water.

A grey whale leaped into the air. The massive body shot towards them.

It was right in their flight path.

'Take her up!' screamed Anawak.

The motors gave an agonised roar and Danny fell backwards as the plane jerked up. Anawak caught a glimpse of an enormous head covered with scars, an eye, a mouth. A powerful blow rocked the cabin. A torn mess of broken linkage replaced the right wing where Danny had been standing. Delaware screamed. The pilot was screaming. Anawak screamed. The sea rushed towards them.

Something hit him in the face. Icy-cold.

There was a droning in his ears. The high-pitched screech of tearing metal.

Spray.

Dark green.

Then nothing.

FIFTY METRES FURTHER DOWN, the onboard computer steadied the cylindrical body of the URA. The robot began to track the nearest whale. Not far away, barely visible in the gloom, others came into view. The electronic eye of the URA took note of them, but the computer wasn't interested in optical data. Other functions took precedence.

The URA's optical sensors were impressive, but its real strength lay in its audio capacity. This was where the inventor had shown a flash of true genius. The robot's acoustic technology allowed it to follow a whale for ten to twelve hours without losing track of it, no matter where it went.

The URA's hydrophones – four sensitive underwater microphones – didn't merely capture the whalesong: they could also determine its source. The hydrophones were fixed at intervals around the robot's case, so that when a whale emitted its high-pitched whistle, they received the sound sequentially. No human ear could register the tiny time delays, the rise and fall in volume: only a computer was capable of that. The noise arrived first at the hydrophone nearest to the whale, then reached the other three in turn.

From there, the computer created a virtual space, assigning coordinates to the source of the sound. Gradually the digital environment filled with positioning data on the whales. The co-ordinates shifted constantly as the animals moved. Now the inside of the computer held a virtual copy of the pod.

Lucy was also emitting sounds as she disappeared into the depths. The computer's memory contained extensive information on specific noises made by whales and different species of fish, as well as the calls of individual animals. The URA searched its electronic catalogue for Lucy but couldn't find her. It automatically created a new entry for the sounds coming from her co-ordinates and compared them to other groups of coordinates, classifying all the surrounding animals as greys and accelerating to two knots to get closer.

Having located and identified the whales, the robot began its optical analysis, which was every bit as thorough as the acoustic diagnosis. Fluke patterns and shapes were stored in its memory, as well as fins, flippers and other identifying features of various whales. This time the computer was in luck. The electronic eye scanned the flukes pounding through the water ahead and identified one of the tails as Lucy's. The URA had been programmed with extensive data on individual whales that had been involved in the attacks. Now it knew which animal was the focus of its attention and changed course by a few degrees.

Whalesong allowed the animals to keep in contact with each other over distances of more than a hundred nautical miles. Sound waves moved through the water five times faster than they did in the air: Lucy could swim as fast as she liked and in any direction, but the robot would never lose her now.