The Watcher pointed to a door. It led Sarah into another corridor that wrapped round the side of the building. Unfortunately it was on the same side of the auditorium as her box, which put it on the opposite side to the woman Sarah had seen slip out of her seat.

She could hear the orchestra switch from tuning to overture as she half-ran, half-skidded along the corridor’s carpet. She turned a tight corner just in time to see Knox and Bennett disappear through another door.

‘Wait!’ she called out, stopping them in their tracks.

Then, when she reached them, she told them what she’d seen.

‘Where does the staircase lead?’ Knox asked when she was done.

‘Outside, and backstage.’

A minute later Sarah was making her way back up to her box to apologise profusely to Lady Balcairn and pretend to enjoy the rest of the evening while also being sure someone was trying to kill a man seated less than twenty feet from her; Watchers and police were being despatched to every exit; and Knox and Bennett were sprinting round to the other side of the auditorium.

Knox thought again how Laing had made the right call about this evening, and he’d ignored him. He’d warned Knox the ballet would be an unmanageable security risk. Knox hadn’t listened, and now here he was, racing to stop a foreign head of state being killed live on national television.

Then he felt something else. Something he’d almost forgotten about—a tingle in the old scar across the top of his head. It used to throb whenever he pushed himself too hard in the maelstrom of action. Now it reminded him how long it had been since he had, and he realised how much he missed it.

It was a sudden, sharp echo of the past. A painful sensation, but also vital, energising, and he didn’t want to let it go. So, as well as chastising himself again for not paying attention to Laing, he decided that, whatever was about to happen, when it was all over he was going to talk to Holland about spending fewer hours sitting behind his desk or on planes and more time out in the field.

‘You reckon she’s our target?’ Bennett asked as they shouldered open another set of doors that led into another stretch of corridor.

‘It’s the only lead we have,’ Knox replied. ‘And it’s very clever. No one is supposed to know Kosygin’s here, and it’s not like we could strip search the whole audience.’

They finally reached the door they’d been told would take them backstage, but as Knox went to open it, Bennett put her hand over his, stopping him for a moment.