Somewhere overhead, there was a violent detonation.

Throughout the store, floors shook, objects vibrated off shelves, and employees looked around uneasily. In the Decorative Accessories department, racks of ugly vases were reduced to uglier shards. Here and there, patches of plaster fell from the ceiling or flaked off the walls, and a fine dust filled the air. The electric lights flickered and died.

Isobel clutched at the railings as the building swayed. In the pits below, the tentacles thrashed to and fro.

"I seem to be making a habit of this," Victoria said. "But I fear I must leave at once."

She squeezed past Isobel and made a run for the door. But Isobel hadn't been more than momentarily distracted. Before Victoria was halfway to the exit, she'd been rugby-tackled and dragged to the floor. To her horror, the walkway was flexing slightly under her, and the appearance of scales could no longer be discounted as a trick of the light; the steel had become dark green, leathery, and very obviously the skin of some creature.

Isobel must have lost the syringe, Victoria decided, or by now it would have been driven into her leg. She grasped the railing and pulled, but Isobel's grip didn't slacken. Not only that, but the floor itself felt as if it was pushing against her. Escape was out of the question, and it was surely only a matter of time before reinforcements arrived. She wrapped her arms and legs round the railing and willed herself to be immobile.

"You will not be harmed," Isobel said, trying to pull her free. "Please do not attempt to resist as this may also be detrimental to your health and safety."

The walkway shuddered and twisted. For one heartstopping moment Victoria and Isobel were both hanging over a seething mass of tentacles, with only Victoria's grasp on the railings between them and being digested. Then the walkway righted itself again.

"Is this your choice?" Victoria asked. "Surrender to you or be thrown to the monster?"

"Your continued safety is our highest..." Isobel stopped. Victoria couldn't see her face, and her voice lacked its usual expressiveness, but there was a definite air of confusion. "Our second highest priority."

"Do tell me. Whatever is your highest priority?"

"Damage limitation."

"Damage limitation?" Victoria repeated. It could be her imagination, but Isobel's grip seemed to be weakening. "Please, tell me more."

"In the event of fire, please follow all instructions given by members of staff," Isobel said. "Do not attempt to retrieve personal possessions."

"Fire? You mean the shop's on fire?"

"Please follow all instructions given by members of staff—" Isobel groaned, whispered "burning," in something a lot closer to her normal voice, and let go of Victoria altogether.

Victoria pulled herself upright and looked round. Isobel was on all fours, shaking her head as if dazed.

"For safety reasons we should leave this area at once," Isobel said, her voice wavering. "Come with me, now."

The walkway rocked again.

Victoria sighed. She could probably make her escape now, but that would leave Isobel in danger. It looked as if the only way to get both of them out was to surrender, or at least pretend to.

"Very well," she said, and held out her hand.

It took Isobel several goes to get up, and once she was on her feet she needed to lean on Victoria for support. Her face was flushed, her hands hot.

Victoria let Isobel lead until they were back in the basement area, at the foot of the staircase. Here, too, the appearance of a normal shop was beginning to crumble. Cracks in every surface were oozing the same sort of purple liquid she'd seen before. A rash of fire alarm buttons, instructing users to break the glass in an emergency, had spread across the walls like chickenpox. The lights had gone out, and dark smoke obscured the skylight, but emergency lights exuded a sickly greenish glow.

Climbing the steps to ground level seemed to take an age. Several times Isobel swayed and nearly fell, or leaned against the wall gasping for breath. Once or twice she tried to talk, but didn't manage to come out with anything coherent. Once they reached the door that led back into the shop, Victoria tried to open it, only to find that Isobel hadn't given up.

"Not right," Isobel said, trying to drag her further up the stairs. "Not where we need you."

"Isobel, you're sick. You need help."

"Where we're going. Not that way." Isobel's grip on Victoria's wrist tightened painfully.

"You're hurting me! Let me go!"

Isobel's grip didn't slacken, and she tugged at Victoria with renewed strength. Victoria tried to break away, but once again found herself at a disadvantage. She didn't want to risk hurting Isobel, but it was all too plain that Isobel didn't feel the same inhibition regarding her. Isobel's fingernails dug painfully into her, and she couldn't help screaming.

Two or three steps further up, Victoria realised that someone had heard her. There were footsteps approaching briskly from further up. Isobel came to a halt and seemed to be listening.

"Victoria Waterfield," Gia said, as she came down the stairs towards them. "I'd know that scream anywhere."

Relief surged in Victoria's mind, followed closely by exasperation. As Gia came closer and Victoria got a clearer view of her, this gave way to unease. Gia looked paler than usual, which Victoria hadn't previously thought possible, and her clothes were covered with plaster dust. A pricing gun dangled from her right hand; it looked as if she might drop it at any moment.

"What happened to you?" Victoria asked. "Are you all right?"

"Physically, yes," Gia said. "Mentally, not so sure."

Isobel chose that moment to try and force Victoria up the stairs once more.

"Can it wait?" Victoria asked. "Isobel's in a bad way. We need to get her out of here."

"I'll try."

Isobel, even possessed, couldn't manage to resist both of them, and in very little time all three were back in the sports department. Apart from the green emergency light, it looked more or less undamaged. There was no sign of anyone else. All the checkouts were deserted, the doors unguarded.

"Where is everyone?" Victoria wondered.

"Upstairs, trying to save what they can," Gia said. She had to raise her voice; Isobel was struggling and kicking in their grip. "Like insects."

"You haven't seen Jamie, I suppose? Or Samantha?"

"Samantha and Zoë were going to cause a diversion. That explosion was probably it. You should see upstairs. Ceilings down, water and purple goo everywhere."

"Zoë's here too? I suppose she came in..."

"With me. It seems ages ago." Gia stopped, her expression growing vacant.

"Gia? What's the matter?"

Gia pulled herself together. "Never mind."

They'd nearly reached the main door when Victoria caught a movement out of the corner of her eye. She turned round. Jamie was standing a little way away, a pricing gun in his hand. He, too, looked slightly feverish.

"You two ladies will come with me," he said.

"Victoria, try to get Isobel out of here," Gia whispered. She let go of Isobel's arm, and raised her own pricing gun.

"Put that down," Jamie said. "You aren't authorised to use it."

Luckily, Isobel seemed to have exhausted her strength earlier. Victoria found that despite Isobel's resistance, she could coax her closer to the door.

"Keep back, or I'll shoot," Gia's voice said from behind her, sounding almost scared.

"In the interests of health and safety," Jamie's voice countered, "You should not attempt to use that equipment."

"Don't come any closer..."

There was click, a rush of air, and a gasp. Victoria turned round, to see Gia lying on her back, a dart protruding from her arm. Her pricing gun lay beside her, unfired. Jamie was bending over her.

"I'm sorry," she gasped. "Get her out of here..."

Jamie took hold of her legs, and began to drag her away. Isobel chose that moment to make her own break for freedom, and for the next minute Victoria was occupied in struggling with her. But Isobel's strength seemed to be fading fast; quicker than she'd hoped, Victoria had reached the door, and pushed it open. At first glance, she couldn't pick anyone out of the crowd that met her eyes. Then she saw the Doctor — her Doctor — and pretty much fell into his arms. UNIT men were rushing past her. Others were attending to Isobel.

"Doctor!" she said, as soon as she could speak. "Gia's still in there, and Jamie. He shot her with some sort of dart."

"You mean he's under the store's influence?"

"Yes. Isobel, too. I'm sure it's killing her." She looked round for the first time, to see Isobel on a stretcher. Liz was bending over her, hypospray in hand.

"I've sedated her for now," she said. "We'll keep her under medical supervision."

Four soldiers emerged from the building, with Gia on another stretcher. Nyssa was with them, looking put out.

"He ran off," she complained. "I couldn't get close enough to him." She leaned over, and plucked something from Victoria's sleeve. "You were lucky. He nearly got you, too."

"What?" Victoria asked, baffled.

Nyssa held up a dart. "It was sticking in your sleeve, right here." She tapped one of the thicker parts of Victoria's rolled-up sleeves. "Anywhere else, and you'd be like those two now." She nodded at Gia and Isobel.

The Doctor took Victoria's hands in his.

"Now, Victoria," he said. "You're the only person who can tell us what's going on. So, please, let us know everything that's happened."