Yozo Sasada. The Aum Shinrikyo member who’d released sarin in the subway carriage her husband Hiko had been travelling in on the morning of 20 March 1995. Sasada had also been accused, but not convicted, of participating in the murder eight months previously of the Chijimatsu family, carried out by feeding sarin into the ducting of their air-conditioning system. The court hadn’t been able to identify any motive for Aum Shinrikyo to go after the Chijimatsus and had eventually dropped the charge.

Kodaka had decided to delve into Sasada’s background. As well as the background of Mitsugi Chijimatsu, head of the family. And there he’d found … Nishizaki.

Mitsugi Chijimatsu was a corporate lawyer who’d advised the targeted company in several failed takeover bids by the Nishizaki Corporation over the years. There was no hard and fast evidence on the point, but Kodaka clearly suspected this had made him a thorn in Nishizaki’s flesh.

As for Yozo Sasada, Kodaka’s researches established that before joining Aum Shinrikyo he’d worked in some junior capacity at a company part-owned by Nishizaki: Quartizon.

The circle was complete, a circle Kodaka had only ever seen part of but was now clear and stark in Wada’s mind. Peter Ellery; Peter Evans; Shitaro Masafumi; Hiroji Nishizaki; Yozo Sasada; Peter Driscoll. And in the middle of the circle: sarin.

Many of Wada’s fellow passengers were asleep by now. But she was alert in every fibre of her being. There was a connection. There was a thread woven over forty years. And her own husband’s death was part of the fabric.

She didn’t know what it all meant. She could see the shape of the secret, but not its pattern. Somehow Martin Caldwell had seen it too. He’d reasoned his way to the answer, or something close to it. And now he was at best a fugitive from Nishizaki, at worst another of his victims.

Wada could risk joining the list of those victims. Or she could run and hide and hope to be forgotten. But what would that achieve, for a solitary, newly unemployed forty-seven-year-old woman? Survival, obviously. But a survival full of regrets. She didn’t want that.

And she wasn’t going to have it.