For a moment Knox entertained the possibility that Kosygin had, in fact, decided to spend the evening in the Soviet embassy, and he and Bennett could combine hunting for a soon-to-be frustrated assassin with watching at least a few minutes of Nureyev’s performance. But then a set of the large glass front doors opened and Kosygin finally made his entrance, wearing a tuxedo and flanked by three other men. As he was guided up to the grand tier by an usher, any last drops of hope and levity instantly drained out of Knox.

‘That’s that, then,’ he said, checking his watch.

If the assassin wanted to make the biggest impact and create the most terror, Knox was sure they’d strike when Nureyev was on stage and people all over the country were watching on the BBC. The curtain was due to go up in five minutes, and Card Game was twenty-five minutes long, which meant they had half an hour to stop a murder.

Sarah Holland never let her nerves get the better of her, but knowing she was very probably walking into the same building as a killer gave her arrival at the Royal Opera House a rather different tone to her usual evenings at the ballet. She maintained her composure as she mingled her way through the entrance hall, stopping to embrace acquaintance after acquaintance. But all she wanted to do was get to her box as quickly as possible.

She delicately excused herself from the last, old son of a banking dynasty who seemed to think that the sizeable donation Sarah had persuaded him to give to Great Ormond Street children’s hospital a few months ago gave him permission to aggressively compliment her pink satin Christian Dior gown, and made her way up the wide staircase to the grand tier a few minutes before the warning bell rang. She felt a mix of hope and apprehension building inside her—hope for what might still simply be an evening of incredible human artistry and athleticism, and apprehension for what might turn it into something even more dramatic.

Sarah’s seat was in the fifth box from the right-hand edge of the opera house’s towering proscenium arch. It was her favourite spot to watch the ballet and opera from, as you could actually see the performances from it rather than just be seen by the rest of the audience during them.

‘Darling,’ a very loud voice said as she stepped inside. ‘I was wondering when you were going to get here.’

Sarah had forgotten that one of the people she’d be sharing her evening with was Lady Jane Balcairn, another person she regularly charmed into giving significant amounts to good causes. Lady Balcairn was also an incorrigible gossip who only stopped talking when she was swallowing champagne or watching ballet.

‘Good evening,’ Sarah replied, a practised smile matching the calm tone of her voice.