She finished filling in the form and looked out of the windows of the jumbo jet. They had chased the sunset west across the Atlantic, and now it was firmly dark blue night out there. Streetlights formed a glittering web when they passed above small towns. Over bigger cities, the lights seemed to be enclosed beneath a dimly glowing bowl that held them trapped, the highway lights leading away from them like chains of refugees. Somewhere out there, her target. Watching TV, eating dinner, reading a book, talking to her lover, gossiping on the phone, composing e-mail. Whatever it was, she wouldn’t be doing it tomorrow. Not if the woman was successful in her mission.

She turned away from the window and pulled her paperback novel out of the seat pocket. She opened it where she had carefully dogeared a page to mark her place and carried on reading Northanger Abbey.

A change in engine note signalled the start of the descent into San Francisco. It was a sign she noted with relief. A transatlantic, transcontinental flight was quite long enough for her body to feel permanently realigned into the shape of the aircraft seat. That might be just about bearable in first class, but back in anonymous economy it provoked the irresistible fear that she might never walk properly again. The woman stretched her spine, thrusting shoulders back and chest out. The sleeping man next to her snorted and shifted in his seat. Thankfully, he’d been like that for most of the flight. She never liked talking on public transport unless she had instigated the conversation, usually for professional reasons.

She couldn’t believe how quickly she cleared immigration. It had been half a dozen years since she’d last set foot in America, and her abiding memory of arrival had been spending the thick end of an hour shuffling forward foot by foot in an endless queue that snaked across the concourse while sadistic immigration officers with faces impassive as hatchets questioned every new arrival. As she collected her luggage, she wondered idly what had brought about the change. It couldn’t be that the Americans had become less xenophobic or less paranoid about terrorism, that was for sure, especially after Oklahoma City. She only had to think about the drop in the numbers of American tourists to Britain in the wake of the IRA’s abandoning of their precarious ceasefire.

Slinging her suit carrier across her shoulder, the woman headed for the taxi rank and gave the name of the hotel where she hoped a room would be waiting for her. Even though she’d been up all night, she feared that sleep would abandon her as soon as her head hit the pillow. It didn’t matter. She had time. According to her briefing, the best opportunity she’d have wouldn’t come before six in the evening of the following day.

She’d heard about the fog rolling in across the bay in the late afternoon, but she’d never quite believed it could be so tangible a phenomenon. She sat among the Sunday tourists in one of the Fisherman’s Wharf cafés and watched the bank of fog envelop the rust-red curve of the Golden Gate, leaving the twin towers stranded above and below. She stirred the last inch of her cappuccino. It had been about the only thing she’d recognized on a list of beverages. They didn’t have iced mocha latte in the coffee bar where she picked up her morning carton of steaming pale brown liquid that smelled mostly of its polystyrene container. She supposed this was what they called culture shock.