'What is phosgene?' asked Bolt.

Cavendish turned to face him. 'It's a component for pharmaceutical products, and it's more commonly used name is mustard gas.'

Bolt was confused. 'Mustard gas? The stuff they used to use in the First World War? And Roy Brakspear was able to order it? Just like that?'

'It's a legal product, Mr Bolt, manufactured in Germany. And we're authorized to import it. This quantity is unusually large, but it won't have been queried because the company has been dealing with us for years.'

'When was this order placed?' asked Mo.

Cavendish typed a command. 'Seven a.m. on Monday morning, so he must have done it remotely. I spoke to Roy about two hours later and he didn't say anything about it.'

'Is there time to cancel the order?' snapped Bolt.

Cavendish typed another command, and Bolt heard him swallow. 'No,' he said quietly. 'According to the system it was picked up at the factory in Germany at two o'clock yesterday afternoon. It must have been a rush order, but I've got no idea who it could be for. Roy hasn't listed who the end user is.'

He didn't need to, thought Bolt. The end user in this case was going to be Hook. But what the hell was he going to do with a lorry load of mustard gas?

'What would happen if the gas was released?' he asked Cavendish. 'What sort of damage would it do?'

Cavendish turned in his seat so he was facing Bolt. He looked stunned. 'But who's going to release it?'

'Just answer the question.'

'It depends on the weather conditions and how it is released. It's carried in light steel cylinders and you can't simply blow them up because that would render the gas ineffective. However, if there was very little wind, the gas was dispersed in a crowded area, and the people who carried out the dispersal somehow managed to break the valves on the cylinders simultaneously without damaging the phosgene, then…' He paused, and when he spoke again, his voice cracked a little. 'I dread to think. The death toll would be hundreds at least. Possibly even thousands.'

Bolt looked at Mo, whose face was draining of colour. Bolt was shocked himself. He knew what Hook was capable of. They both did. There were only two reasons why a man like him could possibly want something as lethal as this. Either to blackmail some other organization, the government perhaps. Or to commit a terrorist act.

Bolt forced himself to remain calm. 'We need to locate the load urgently,' he said. 'How do we do it?'

'I don't know which driver we used. Roy didn't fill his name in on the order form. There'll be a signature on the paperwork though. That should tell us who it is.' He began typing again, and a copy of the German invoice reappeared on the screen. 'There's the signature,' he said, pointing to an illegible squiggle in the bottom right-hand corner. He examined it closely, shaking his head. 'I'm afraid I don't recognize it.'

'What the hell is this?' demanded Mo, his face red with anger. 'You don't know who the hell's driving a deadly cargo on your company's behalf?'

Bolt could understand his friend's reaction. He had a wife and four children at home in London. If Hook was plotting a terrorist outrage then it was a fair guess that the capital would be the target, which put a hugely personal slant on the case.

'It's not my fault if Roy didn't fill out the information,' said Cavendish defensively.

Mo wasn't mollified. 'What about the checks and balances?' he asked. 'You should have known about it. You're a director of the company, for God's sake.'

'Roy's a director, too. He's in charge of the bloody checks and balances. How was I to know he'd do something like this? I can't understand it. What the hell did he think he was doing?' Cavendish put his head in his hands and stared down at the desktop.

Bolt put a hand on his shoulder. 'OK, Mr Cavendish, no one's saying it's your fault. But we really do need to locate the lorry carrying this load.'

Cavendish slowly lifted his head and looked up at Bolt with frightened eyes. 'That's the problem, Mr Bolt. We don't have any of our own drivers. We use agency ones, and they come from all over the place. That bloody gas could be anywhere.'

Mike Bolt was an optimist. He'd had some hard, hard times – the death of Mikaela being the hardest of all – but he remained conscious of the fact that if he kept a level head and rode the punches thrown at him, eventually he'd come through the other side, and things would get better. Because if you let them, they always did.

But at that moment he was having to work hard to keep his spirits up. Finding the mustard gas and, by extension, Tina and Jenny Brakspear was looking like an impossible task. In Hook, they were up against a highly professional operator who'd only remained at liberty for so long because he kept ahead of the game. But it was still possible, he told himself. It was just a matter of staying calm and working through the leads they had, and for that they needed resources.

It was twenty to four when Bolt stepped outside into Cavendish's back garden, leaving Mo inside with him. He dialled Big Barry's home number. The case had changed dramatically now that national security was threatened, and Bolt needed his boss's help.

Big Barry still sounded asleep when he answered the phone, but that didn't last long. 'How could this happen?' he demanded when Bolt told him about the mustard gas.

'All too easily, by the sound of things. Obviously, the important thing is to find the bloody stuff. Cavendish is in a bit of a state of shock.'

'I'm not bloody surprised. He'll be in even more of a one if it gets let off and it's his firm that's responsible for it. How bad could it be if it's released?'

Bolt told him.

'Christ.' There was silence on the other end of the line as Big Barry took the information in. 'I'm going to have to get the director involved. The PM's going to have to know about this as well. This is government-level stuff.'

'I know,' said Bolt, moving further into the garden. 'Cavendish has given us a list of agencies in the UK and Europe that Mainline have used before to hire drivers. But there are a lot of them. Eighteen altogether. And as Brakspear was trying to hide what he was doing, it's possible he could have gone to someone else.'

'How do we know that he even hired a driver? What's to stop him sending one of the kidnappers over to pick up the order?'

'I asked Cavendish about that. All drivers carrying hazardous goods have to have something called an APR licence, which has photo ID on it. They give them out to plenty of people, but it's unlikely the company manufacturing the mustard gas in Germany would have given the order to someone who didn't have a valid one. One could always be faked, I suppose, but my guess is Hook's going to try to intercept the load somewhere between the factory and the final destination, which is a secure facility that Mainline have just outside Cambridge.'

'If he hasn't done it already.' Big Barry sighed. 'OK, email me that list and the name of the company in Germany. I'll get resources lined up to contact everyone, but it's all going to take a while at this time of the morning.'

'There might be a quicker way,' said Bolt.

'What?'

'I'm pretty sure Brakspear was being imprisoned in his home shortly after Jenny was kidnapped. He was there on Monday because Tina and Cavendish both told me they called him at different times, and he was there when Fallon turned up yesterday morning. So it's possible he called the agency to hire the driver from his home phone. If we check the records, we might be able to find out who it is.'

'Good thinking,' said Big Barry, suddenly sounding a little happier. 'Good work, old mate. I tell you: if we stop this stuff falling into the wrong hands, it'll be a real result for SOCA. A high-profile one, too.'