"But this girl isn't Lexie Madison. She's an identity thief; I could go down to Fraud in the morning and find you hundreds more just like her. There is no Lexie Madison. You and Mackey made her up."

His hands had tightened on mine. "I know," I said. "That's sort of the point."

The corner of Sam's mouth twisted. "Like I said. The man's mad."

I didn't exactly disagree with him. I've always thought one of the reasons for Frank's legendary fearlessness is that, way deep down, he's never quite managed to connect with reality. To him every operation is one of those war games the Pentagon plays, only even cooler, because the stakes are higher and the results are tangible and long-lasting. The fracture is small enough and he's smart enough that it never shows in obvious ways; but, while he's keeping every angle covered and every situation beautifully, icily under control, some part of him truly believes he's being played by Sean Connery.

I spotted this because I recognize it. My own border fence between real and not-real has never been all that great. My friend Emma, who likes things to add up neatly, claims that this is because my parents died when I was too young to take it in: they were there one day and gone the next, crashing through that fence so hard and fast they left it splintered for good. When I was Lexie Madison for eight months she turned into a real person to me, a sister I lost or left behind on the way; a shadow somewhere inside me, like the shadows of vanishing twins that show up on people's X-rays once in a blue moon. Even before she came back to find me I knew I owed her something, for being the one who lived.

This was presumably not what Sam wanted to hear; he had enough on his plate without adding several new flavors of weird into the mix. Instead--it was the closest I could get--I tried to tell him about undercover. I told him how your senses are never quite the same again, how colors turn fierce enough to brand you and the air tastes bright and jagged as that clear liqueur filled with tiny flakes of gold; how the way you walk changes, your balance turns fine and taut as a surfer's, when you spend every second on the shifting edge of a fast risky wave. I told him how afterwards I never shared a spliff with my mates or took E in a club again, because no high could ever compare. I told him how damn good at it I was, a natural, better than I'll be at DV in a million years.

When I finished, Sam was looking at me with a worried little furrow between his eyebrows. "What are you saying?" he asked. "Are you saying you want to transfer back into Undercover?"

He had taken his hands off mine. I looked at him, sitting across the sofa with his hair rucked up on one side, frowning at me. "No," I said, "that's not it," and watched his face clear in relief. "That's not it at all."