Government Communications Headquarters appeared out of the landscape like a vast space-age tumulus—a stadium landed among the Cheltenham suburbs, with half a mile of tarmac between its secrets and the rest of society. The processing and monitoring hub of a surveillance empire, GCHQ and its network of satellites and ground stations eavesdropped on every square inch of the globe. It was classified to an extent beyond anything in MI6, but then, it was more effective.

They passed the first discreet security camera, then reached an entrance ringed by barbed wire. A guard checked Taylor’s pass, peered at Kane, raised a barrier. A few seconds later they paused as another camera stared through the windscreen, and then two metal posts sank into the ground and they drove down a ramp into the visitors’ car park.

Kane had only been in once before, and that was more than ten years ago. A familiar curiosity returned, with a familiar caveat: being let into secrets is one thing, being let out, another. The car park had its own security center, where they handed in their phones. Kane signed a visitors book and took his temporary pass. They received a guide, a young woman in a cardigan and heels who said “Follow me” and led them up a flight of stairs into the largest secret intelligence headquarters outside of the US.

A veneer of normality was now allowed to present itself. Beyond an atrium with sofas and potted palm trees, the HQ opened out into two levels of communal seating lit through a glass ceiling. A floating walkway circled the space, ivy creeping up its supporting columns. The architectural theme, without trace of irony, was openness.

Kane had been given a tour after the terrorist attacks of July 2007, in the spirit of encouraging cooperation between the services. Barriers were being forced down between the traditionally rivalrous MI5, MI6, and GCHQ, with the idea that joint teams could work together and share leads. GCHQ was the last of the three to emerge from the shadows, and some were surprised to discover it was twice the size of MI5 and MI6 combined. It emerged reluctantly, highly protective of its techniques, not wanting to compromise its ingenuity.

Kane hadn’t expected ever to return. Now the four of them strode through the headquarters, with their guide angling her face toward various cameras in order to open doors. No one paid them any attention. Plenty of staff still wore casual clothes: not quite the vibe of ten years ago—no shorts and flip-flops—but the Starbucks was still surreally there, and some staff carried gym bags and squash rackets. And in the empty center of the circular building was the garden with its memorial to dead colleagues, a reminder that in the new era of warfare, there were software engineers on the front line too, that hackers sacrificed their lives with increasing frequency.