Security-camera footage of Yulia Skripal's movements through the Moscow airport on March 3, a day before the poisoning, showed a slender woman with an erect bearing, shuffling through check-in with other travelers. A fold of auburn hair fell over her pale face. That morning Yulia shared a video on social media, a gyrating dog alongside the caption: Dance like no one is watching. According to discoveries by British intelligence, later made public in a submission to NATO, “cyber specialists” working for the GRU had been snooping in Yulia's e-mails as far back as 2013. Furthermore, according to a person with knowledge of the investigation, Sergei's e-mails were also under surveillance during that period. “They would have known Yulia was coming,” a senior government source told me.

4. A City on Lockdown

Like Ink, liquid novichok can transfer by contact from surface to surface, fainter each time; it is colorless, odorless, and deteriorates slowly. Shortly after the Skripals arrived at the hospital, a local cop, Nick Bailey, came into contact with the poison and had to be admitted for treatment as well. Later I would learn that this particular novichok had most likely never been tested on humans. It all meant that in Salisbury, authorities were now trying to cope with an invisible, spreadable, untraceable poison that might be smeared anywhere the Skripals had been on March 4.

When I first arrived in the city, I visited the riverside pub where the Skripals had had a drink before their collapse, pressing my face to its plate-glass door as investigators slow-motioned around inside wearing hazmat suits. Three hours later, a public-health bulletin went out, advising anyone who'd been to the pub to disinfect their phones, wash their clothes, and so on.

The bench where the Skripals had been overcome was enclosed by ribbons of don't-cross tape. Brightly colored forensic tents popped up like spring flowers. Reporters skulked, and the spook-watchers among them noticed the movements of unmarked cars known to be favored by British intelligence.

Over on Christie Miller Road, Skripal's property had been cordoned off, so that neighbors had to wave a special pass at police to come and go. I walked a perimeter around No. 47, as close as possible, in the company of Boris Volodarsky, the intelligence historian and former GRU man, who was spending the day with me in Salisbury. A suited and cardiganed 63-year-old, his face partially obscured by a brushy mustache and aviator sunglasses, Volodarsky was about the most conspicuous man in town that afternoon. But he had no reason to steal around in the shadows, not anymore, and instead he turned an operational eye on the scene, trying to identify the shadows that might've been useful to others.

This operation would've called for a large team, Volodarsky said—Russian illegals, he thought, arriving in the country over the course of weeks to study the local minutiae: “When lights were switched on and off, did neighbors look out their windows?” Crucial to any such plan, Volodarsky said, would be settling on a where—someplace they could be certain Sergei Skripal would be—and a when. The application of this poison would have been skilled work, technically complex. “[The nerve agent] will burn through normal hazmat suits,” a senior source from the UK government told me. “You need time and you need cover.”

“You think to yourself, it’s either a well-done job or a badly done job,” one former GRU agent said, about an attempt on his life. “If it’s one, you’ll die. If it’s the other, you’ll survive.”

From Christie Miller Road, we drove two miles to a cemetery in Salisbury, a wooded place home to wild ring-necked pheasant, deer that liked to eat the graveside roses, and for the time being several police cars. Investigators believed that on the morning of their poisoning, Sergei and Yulia had come to this cemetery to visit the graves of Liudmila and Alexander. Police, still guarding the site, took our names and let us through, and when Volodarsky was close enough to Alexander Skripal's headstone, he read the inscribed dates. He pointed out that March 1 would have been Alexander's 44th birthday.

It felt a fair assumption that this was why Yulia had arranged to visit that particular weekend. (A when.) And if Yulia was due, Sergei would surely be in Salisbury to bring her back to his home. (A where.) According to the UK's national-security adviser, the highest concentration of novichok was detected on the handle of No. 47's front door. A senior government source later confirmed for me that it was the outward-facing handle, and that Sergei and Yulia had each taken in the poison through the palms of their hands. Alistair Hay, a chemical-weapons expert, explained that because of the thickness of the epidermis layer here, “uptake through the palm is some 20 to 25 times less efficient [i.e., slower] than, say, application on the cheek.” Time—hours, potentially—for any assassins to flee or melt away.