Samantha wasn't giving up without a fight, but in the grasp of two store detectives, both bigger than she was, her options were limited.

"Get off me!" she shouted at them, more for something to shout than because she thought it would do any good.

"Let her go," Gia's voice added. "Or I'll blow your brains, such as they are, across the room."

The grip on Samantha's left arm suddenly relaxed. A few seconds later, so did the one on her right. She pulled free, to see that Gia was holding something to one man's head, while Zoë was efficiently wrestling the other guard to the floor with a succession of judo moves.

"Put your hands on the wall," Gia said. "And no sudden movements."

The guard she was standing behind did so.

"Sam, keep this on him," she said, passing a piece of copper pipe to Samantha with a wink. While Samantha kept the end of the pipe pressed against the man's head, Gia frisked her captive, relieving him of a syringe, a baton and a pair of handcuffs. The latter she promptly snapped round the man's own wrists. With the baton in her hands, she turned to his colleague. By now, Zoë was pinning him to the ground and had her arm locked around his throat, but he showed no signs of giving up the struggle.

"I don't think he's human," Zoë said. "If he was, he'd be dead by now."

"That's a nuisance. It means we don't know how hard to hit him to knock him out."

"You tie him up then, while I keep him pinned."

Gia retrieved the guard's handcuffs, and with Zoë's help secured him. Then she snatched a handful of scarves from a nearby display, tied both guards' feet, and gagged them.

"So much for them," she said. "Now, Sam, what's been going on here?"

"Long story," Samantha said. "But in short, this shop's taken over Jamie and Isobel. And it isn't a shop, it's some sort of creature."

"Thought so," Zoë said. "Have you seen any trolleys moving about by themselves?"

"Nope."

"Oh. I'd have expected that."

"Anyway, Victoria and I got split up. I don't know where she is, but you'll know her if you see her. She's dressed like a bumblebee, black and yellow stripes all over."

"Why?"

"Dunno. Oh, and you were right about those guards. I reckon they're just clothes full of purple gunk. No insides at all."

"Curious. If the shop can construct humanoid drones, why is it recruiting real humans? We'll have to look into that."

"Leave that for later," Gia said. "Let's get you out of here."

"No way. I'm not leaving until I know Jamie and Isobel are all right."

"Do you know how they were placed under control?"

"With Isobel, I think it was the shortbread she ate. And Jamie started acting funny after one of the shopgirls snogged him."

"Some sort of biological or chemical mechanism, then," Zoë said.

Gia held up the syringe she'd found. "And this must have been meant for you. Right, we'll need to get this to the Doctor and see about an antidote."

"I'll do that," Zoë said, took the syringe, and headed for the main door. A few feet away from it, she clapped her free hand to her head and dropped to her knees.

"What's the matter?" Samantha asked, dashing over to her.

"Silenski implant," Zoë replied, forcing the words out.

"You what?"

"My brain's tamperproofed. And something's trying to tamper with it."

Gia nodded at the waist-high loops of wire that stood on either side of the door.

"Not quite the alarm system I was expecting," she said. "All right. I'll do it. Sam, you try and get Zoë away from the door."

She left the shop, syringe in hand, and returned less than a minute later without it. Zoë was back on her feet, keeping a respectful distance away from the alarm loops.

"Right," Gia said. "Sergeant Benton's going to deal with that for us. Now what?"

"Find the others and get them out," Samantha said.

"That might not be too easy if this whole organism is focused on us," Zoë said thoughtfully. "What we need to do is distract it somehow."

She ran her finger down a nearby store guide.

"Here we are," she said. "Restaurant, top floor. We know how to deal with restaurants."

"Right," Samantha said. "I'm coming with you. Gia, get after Victoria and the others."

"Yes, sir," Gia said, and saluted ironically.

When Victoria had escaped from Jamie, she'd run down the stairs in a blind panic, and hadn't realised until too late that she'd gone down a little further than she'd come up. The area around the foot of the stairs was lit by the same fluorescent tubes as elsewhere, but the light they gave out was yellowish and flickery. Somewhere overhead, she could hear running footsteps and shouting, sounding very far away.

The sensible thing to do would be not to attract attention. Wait for the pursuit to die down, and then go back to the public areas of the store and try to blend in. But Victoria had already tried that, and got nowhere. To her own surprise, she felt inclined to take a few risks.

As she pushed the nearest door open, she wondered if this was how the Doctor felt all the time.

"Excuse me," Gia said.

The young woman at the leather sales counter looked up.

"Hello," she said, in a singsong voice. "My name is ViolettehowcanIbeofservicetoyou. How can I be of—"

"I heard the first time."

The woman stared at her blankly. Gia felt as if she'd just given incorrect input to a computer.

"Tell me," she said, in the slow and clear voice of one addressing voice recognition software. "Have you seen a girl round here? Dark hair, short, dressed in black and yellow stripes?"

"I'm afraid not." The woman leaned forward, her eyes fixed on Gia's. "Can I improve your day in any other way?"

Gia narrowed her own eyes. "Does this shop have an electrical goods department?"

"Third floor, near the lifts. Have a nice day."

"Thank you." Gia turned to leave, and then turned back. "Oh, by the—"

She dived to one side. The dart whipped past her and buried its head in a handbag on a stand behind her.

ViolettehowcanIbeofservicetoyou threw her blowpipe aside, produced a syringe, and emerged from behind the counter, still staring fixedly at Gia.

Gia briefly weighed up the options of running or fighting, and decided to stand and fight. Judging by the examples she'd met so far, this shop didn't endow its drones with very much intelligence, and if she couldn't outthink this one she might as well give up now. She snatched a glove from a table behind her.

"How much is this?" she asked, waving it in the assistant's face. With her other hand she grabbed the arm holding the syringe and twisted hard.

She had hoped that she might make the girl drop the syringe. What she hadn't expected was for the arm to come off altogether, and the assistant, spraying purple liquid, to deflate with an inhuman wail. Her face, frozen into immobility, stared up at Gia from a pile of clothes.

Gia realised she was still holding the arm she'd pulled off. Now she looked at it, it was just a sleeve, with liquid dripping from the severed end. She threw it to the ground, and made for the escalator. To her surprise, she realised she was shaking.

I didn't kill her, she told herself. She was never really alive anyway. It's not as if I meant to do that. And she was trying to stick that syringe in me...

She reached the escalator, leaned on the handrail, and tried to stop her teeth chattering.

Victoria tried to make sense of the scene before her. Like the room she and Samantha had explored before, it seemed to be based around a disconcerting combination of organic and inorganic elements. Unlike that room, there was a clear division between the two.

The door through which she'd entered led onto a narrow walkway, made of steel, lined with railings, and with patches of rust in the same scale-like pattern that she'd noticed everywhere else. On either side of the walkway was a series of pits, some of which contained heaps of everyday items — clothes, furniture, shoes, toys. Purplish protrusions, something like tentacles and something like roots, grew from the walls and floors and spread out over the heaps.

Walking slowly along the walkway, Victoria tried to puzzle out what all this stuff had in common. The heaps appeared to be sorted by content; no-one had thrown a saucepan into the clothing pile. And everything here looked more or less like something this shop would sell. Was it being used as some kind of a template? It didn't seem likely: the quality of the goods here was higher than anything she'd seen above.

As she watched, a chair in the pile of furniture, which several of the rootlike things were touching, cracked and fell apart. A few items above it moved slightly, shifting this way and that, and exposed a cash register, of a different design from the ones this store used.

"Now where did that come from?" Victoria wondered out loud.

"It is the store's policy not to reveal trade secrets," Isobel's voice said.

Victoria spun round. Isobel was standing at the door.

"You are in no position to escape," Isobel continued. "This area is closed to the general public for reasons of safety. I must ask you to leave at once. Please walk slowly towards me."

"Come and get me," Victoria said. There was no point in making things easy for Isobel or whoever was controlling her.

Isobel began to advance onto the walkway.

"For your reassurance," she said, "I should let you know that I have been fully trained to work safely in this environment."

There was something vaguely reflective in one of her hands, Victoria noted.

"Please keep away from the digestion chambers," Isobel continued. "Falling into one of the chambers may be detrimental to your health and safety."

Isobel was now within feet of Victoria. The glittering object in her hand was now clearly visible as a syringe, held ready to be plunged into whatever part of Victoria was most convenient.

"Have a nice day," she concluded.

Gia had reached Electrical Goods with the vague intention of cannibalising a few devices and building something that could help in rescuing Victoria and the others — something to detect lifesigns, perhaps, or to paralyse whatever the store used as a nervous system.

A glance at the technology on display told her it wasn't going to be that easy. Not only was it primitive — she'd expected that — but exceedingly cheap and nasty. And as soon as she pulled out an insulated screwdriver and started dismantling a portable radio, the staff were onto her. Another one of the inhuman assistants, this one a violet-eyed beauty by the name of Moniiqueaskmeifyourequireanything, had hurried up at once.

"I must ask you not to tamper with the goods on display," she began. "It is the policy of this store—"

Gia raised her screwdriver.

"Don't make me use this," she said, sounding rather shakier than she'd hoped. "I don't want to hurt you."

The attendant ignored the threat as if it hadn't been uttered.

"For the convenience of customers, all our staff in this area will be happy to answer questions about the products," she said. "We therefore ask customers not to risk causing unintentional damage."

"What about intentional damage?" Gia asked. "Keep back."

The woman pulled a pricing gun from her belt, aimed it at Gia, and pulled the trigger back halfway. Maybe it was the accompanying whine of power, or the little cluster of glowing red LEDs round its muzzle, but Gia suspected that it could do a lot more than mark her down as a never-to-be-repeated bargain.