It was very early the next morning, just as the eastern sky above The Rope Walk was turning pink, when a small, luminous green ball rolled noiselessly along the middle of the street and stopped outside the Port Witch Coven's house. Sleuth paused for a moment, bouncing on the spot while it took stock. Sleuth was content. It knew it had nearly reached its destination. Since its Master had thrown it out with its Tag, it had faithfully followed not only Jenna's exact footsteps, but also the rhythm of her journey, speeding up where she had done so and stopping where she had stopped. Which is why the green ball was waiting a moment at the very spot where, not so very many hours ago, Jenna had questioned Stanley's sense of direction.

This was how the Tracker Ball operated, and it was extremely effective, although not without the occasional problem. Such as when, late that afternoon, ten feet of choppy sea-water had covered the route that Jenna took earlier in the day, when the tide was out. That had slowed Sleuth down a bit, and getting covered in sand afterward had not helped its progress either. Sleuth knew its Master would not be pleased about the delay and it was anxious to get on with its task. The ball bounced up to the door of the Port Witch Coven, only to feel a sudden urge to leave again. Sleuth was about to bounce away when the door was flung open and a hand shot out and grabbed it.

"Got it!" a triumphant witch yelled. Sleuth was furious. It struggled but the witch held it tight in her grasp.

"Got what, Linda?" Sleuth glimpsed another, older, witch's shocked face as Linda showed off her catch. Her face was white with fright. "Oh, Covens aboveare you trying to get us all killed?"

"What are you going on about?" snapped the younger witch. "You're just cross because you missed that rat earlier. Anywayit's my ball now. So shove off."

"Lindafor Coven's sake, let it go. It belongs to the Master. It's a Tracker Ball on a mission. Drop it right now!"

The witch dropped Sleuth like a hot potato. The ball shook itself and bounced back into the street, then made its way to the door of the Doll House. The two witches watched, fascinated, as Sleuth briefly bounced on the spot and, on the third bounce, squeezed through the letterbox and disappeared inside.

"Shame it's not come for anyone here," said the older witch. "We could have Kept them for the Master. That would have put us in his good books."

"We're never in the right place at the right time, are we?" Linda sighed gloomily as she slammed the door with a loud bang.

"Nicko?" whispered Jenna. "Nicko?"

"Werr?"

"Nicko, there's someone tapping on the window."

" 'S only mad Nurse, Jen. Ger bakoo sleep," Nicko mumbled drowsily from his lumpy bed in the corner of the grubby room.

Jenna sat up in her equally lumpy bed and pulled Lucy's cloak around her. She stared into the gloom, her heart pounding, and listened again. It sounded as though the Nurse was bouncing a ball outside their window. Why? She hadn't looked the sporty type. And then, as the fog of sleep finally left Jenna's brain, she remembered. Sleuth.

Jenna leaped out of bed and immediately fell over the sleeping form of Septimus, who was wrapped up in a blanket on the floor. He did not stir. Slowly, she crawled over to the window, keeping low in the hope that Sleuth would not see heralthough Jenna suspected that it did not matter one bit whether or not the Tracker Ball saw her. It knew she was here.

And then Jenna trod on something softand alive. She opened her mouth to scream but before she could, a hand had snaked over her face and covered the scream. A smell of damp earth filled Jenna's nostrils and two big eyes stared at her.

"Shh," whispered Wolf Boy, who had been lying under the window listening to Sleuth for the last five minutes. "There's a Thing outside. I saw one just like it in the Forest once."

"I know," whispered Jenna. "It's come to find me."

"Do you want me to catch it for you?" asked Wolf Boy, his eyes glittering in the green glow that was shining through the prime on the window. Outside, Sleuth was growing brighter by the second. It had found the quarry; now it was gathering the energy to Tag the quarry. Once Sleuth had done that, it would return to its Master, mission accomplished. From then on, the quarry was Tagged, and its Master would know where to find it.

"Can you catch it?" wondered Jenna, thinking that Sleuth would be far too fast for Wolf Boy.

"Easy." Wolf Boy grinned, his dirty teeth shining a nasty shade of green in the ever-brightening glow. "Watch."

Quicker than a witch, Wolf Boy threw open the window and had Sleuth clasped in his hand in a flash. He slammed the window shut with a bang.

"Get him!" yelled Septimus, sitting bolt upright, his eyes open wide, still in the middle of a dream.

"What?" mumbled Nicko. "Whawhat's going on? Jen? Why's he gone green?"

Wolf Boy looked very peculiar. The bright pulsing light from the trapped Sleuth shone through his hands in a reddish-green glow, outlining the bones as dark shapes beneath his skin. The rest of Wolf Boy was turning a horrible shade of green as Sleuth glowed ever brighter, trying to gather the energy to escape.

The Tracker Ball was furious. It was so near and yet so far, for unless it could Tag the quarry, what use was it to its Master? No more use than a bald old tennis ball, that's what. Sleuth knew all about bald old tennis balls, for it had once been one itself. Sleuth owed everything to its Master, Simon, and it would never let him down. Nothing was going to stop it from Tagging the quarry. Nothing.

However, Wolf Boy was doing his best. His strong wiry hands held Sleuth in a grip of iron, while Sleuth gathered all its energy and slowly but surely began to heat up. It was a risky ploy, but the Tracker Ball was willing to risk the possibility of meltdown.

It would rather liquefy into a pool of rubber than fail its Master.

"Why are your hands green, 409?" asked Septimus, still bleary-eyed and under the impression he was back in the Young Army dormitory with Boy 409.

"Dunno. It's some kind of Thing. Jenna asked me to catch it. So I did. Funny, it's getting quite hot."

"It's Sleuth," whispered Jenna. "Simon's Tracker Ball. He's sent it to find me. What are we going to do with it?"

Septimus was suddenly wide awake. "Don't let it touch you, Jen. He's put a Tag on it. It mustn't touch yougot that?"

"I don't want it to." Jenna shuddered. "Horrible thing."

"If it doesn't touch you, it won't be able to go back to Simon and tell him where you are. So you're still safe. Okay?"

Jenna looked anything but okay. She was pale and trembling and had a green tinge about her.

"Ouch..." Wolf Boy muttered. "Oohah. Ouch!"

"You all right?" asked Nicko.

"Ah ... It's getting hot ... I can'tI can't hold it ... Aargh!" Wolf Boy dropped the Tracker Ball, the palms of his hands burned raw.

Sleuth was glowing so bright it hurt to look at itit was red-hot. At lightning speed it shot over to Jenna, jumped up and touched her arm. Jenna screamed with pain and shock. The ball hurled itself through the window, smashing the glass, and burned its way through the wooden bridge, landing far below in the witches' rotting rubbish heap with a loud hiss. It lay for a moment deep in the pile of tea leaves, rabbit bones and frog heads, and waited until it had cooled down.

Then, triumphantly, it shot out of the rubbish heap, shook off a thick coating of tea leaves and sped away, back to its Master, Simon Heap.