‘Especially not by their sisters!’ Charlie tried to sound as proud as she could of her achievement, to counter Simon’s disapproving tone. ‘I assumed she was heading to Spilling to meet Gibbs after work, though it was a bit early still. Why would she go so early? I hovered in the doorway of WHSmith and made sure to stay well behind her. Every time she looked as if she might be turning in my direction, I ducked into the shop and hid. I could still see her clearly – I had a good vantage point, peering out from behind a big square “Deal of the Week” board.’

‘She could easily have seen you.’

‘Yeah, I know. Luckily, she didn’t. If she had, I’d have said, “Just suddenly had a craving for a can of Dr Pepper, so I came back up from the tube” – a barefaced lie I’d have been quite happy telling. I find it hard to persuade myself I owe her any honesty at all these days.’

‘So what happened?’ Simon asked. ‘She caught a train?’

‘Yes, about ten minutes later, to Cambridge.’

‘Hardly surprising. We know Liv’s got friends in Cambridge – we’ve followed her and Gibbs there before.’

‘Oh, come on, for God’s sake!’ Charlie’s forehead felt hot. So did her palms on the steering wheel. ‘Suddenly you’re all “Yawn, yawn, nothing to see here.” This isn’t personal for you like it is for me, Simon. Imagine if it was your mother behaving like Liv. You’d have to know, wouldn’t you? That’s how I feel. I have to find out.’

‘Yeah,’ Simon breathed. To Charlie it sounded like someone giving up.

She was too embarrassed to describe her elation that was as strong now as yesterday: she’d followed Liv, and she’d completed her mission successfully. As a result, she had new information. Ever since, she’d felt as if there were a helium balloon inside her, giving her a floating-above-everything sensation. Why was it so frowned upon to be noticeably excited once you were a grown-up?

The strange thing about it was her being police. Before she’d transferred out of CID, Charlie had worked on many serious crime investigations, closed plenty of cases, found out useful information that had helped to identify and put away criminals, so why did illicitly following her sister to Cambridge feel like the most exciting thing she’d ever done?