The two men were dressed inconspicuously in jeans, fleece jackets and trainers as they boarded the flight from Moscow to Gatwick. Their names, according to their Russian passports, were Alexander Petrov and Ruslan Boshirov. Both were around 40 years old. Neither looked suspicious.

The plane trundled down the icy runway. In Moscow the temperatures had fallen below -10C, not unusual for early March. In Britain it had been snowing. The pair had brought woolly hats. They had also packed a bottle of what appeared to be the Nina Ricci perfume Premier Jour. The box it came in was prettily decorated with flowers, it listed ingredients including alcohol and it bore the words “Made in France”.

According to the Metropolitan police, the bottle in fact contained novichok, a lethal nerve agent developed in the late Soviet Union. The bottle had been specially made to be leakproof and had a customised applicator. Moscow’s notorious poisons factory run by the KGB made similar devices throughout the cold war.

Petrov and Boshirov were aliases, detectives believe. Both men are suspected to be career officers with the GRU, Russia’s powerful and highly secretive military intelligence service. The GRU is separate from the FSB and sometimes a rival to it. It is the agency the FBI has said hacked Democratic party emails during the 2016 presidential election.

The officers’ assignment was covert. They were coming to Britain not as tourists but as assassins. Their target was Sergei Skripal, a former GRU officer who spied for British intelligence, got caught and was freed in a spy exchange in 2010. They were heading for his home in provincial Salisbury.

Their Aeroflot flight SU2588 touched down at 3pm on Friday 2 March. They were recorded on CCTV going through passport control, Boshirov with dark hair and a goatee beard, Petrov unshaven and wearing a blue gingham shirt. Both were carrying satchels slung casually over the shoulder. According to police, the pair had visited the UK before.

From Gatwick they caught the train to London Victoria station and then the tube to east London, where they checked in to the City Stay hotel in Bow. It was a low-profile choice of accommodation. The red-brick Victorian building is next to a branch of Barclays bank, a busy train line and a wall daubed with graffiti. Across the road is a car pound and a Texaco garage.

The City Stay hotel in Bow, east London, where the two men stayed. Photograph: Toby Melville/Reuters

The plot to kill Skripal was uncannily similar to the operation in 2006 to murder Alexander Litvinenko, the Russian dissident and former FSB officer who was poisoned with radioactive tea. His two assassins, Andrei Lugovoi and Dmitry Kovtun, arrived at Gatwick from Moscow and checked into a Soho hotel. The murder weapon was polonium-210, almost certainly hidden in another small container.

On hostile territory, Boshirov and Petrov operated in the manner of classic intelligence operatives. The next day, Saturday 3 March, they travelled to Salisbury to check out the area, catching the train from London Waterloo and arriving at 2.25pm. They spent nearly two hours in town and left at 4.10pm. CCTV captured them at Salisbury station apparently peering at the departure board, Borishov holding a pair of black gloves.

On the day of the hit, according to detectives, the pair made a similar journey, taking the 8.05am train from Waterloo to Salisbury and arriving at 11.48am. The perfume bottle was probably concealed in a light grey backpack carried by Petrov. From Salisbury station the two men set off on foot. It was a short walk of about a mile to Skripal’s semi-detached home in Christie Miller Road.

CCTV recorded them again, walking side by side along Wilton Road, almost in lockstep, minutes before the attack. They passed a petrol station. At some point they will have turned right, either along a road or up a path that leads through a small wood. At Skripal’s house the Russians smeared or sprayed novichok on to the front door handle, police say.

The moment went unobserved but an image retrieved by police from thousands of hours of CCTV offers further clues. At 1.05pm the men were recorded in Fisherton Street on their way back to the station. They appeared more relaxed, Petrov grinning even. Their main remaining tasks were to dispose of the poison and leave the country.

The two suspects on Fisherton Road in Salisbury at 1.05pm on 4 March. Photograph: Handout/Getty Images

At some point on their walk back they must have tossed away the bottle, which at this point was too dangerous to try to smuggle back through customs. It ended up in a charity bin, which would later have terrible consequences for Dawn Sturgess and Charlie Rowley.

At 1.50pm the suspects were seen at Salisbury station, going through the ticket barriers on their way back to London. Nobody paid them much attention. By the time Sergei Skripal and his daughter, Yulia, were found collapsed on a park bench in the centre of Salisbury later that afternoon, the poisoners were gone.

Seemingly, the GRU plan – executed two weeks before Russia’s presidential election – had worked perfectly. Except it hadn’t. As in the Litivnenko case, the would-be killers had left a trail. Minute traces of novichok were found in the hotel room where Petrov and Boshirov stayed. And against all medical predictions, the Skripals survived.

The visitors were captured on CCTV one more time, at Heathrow airport. It was 7.28pm and both men were going through security, Petrov first, wheeling a small black case. In his right hand was a shiny red object, his Russian passport. Police believe the passport was genuine, his name not. In other words, that it was a sophisticated espionage operation carried out by a state or state entities.

Quick Guide What is novichok? Show Novichok refers to a group of nerve agents developed by the Soviet Union in the 1970s and 80s to elude international restrictions on chemical weapons. Like other nerve agents, they are organophosphate compounds, but the chemicals used to make them, and their final structures, are considered classified in the UK, the US and other countries. The most potent of the novichok substances are considered to be more lethal than VX, the most deadly of the familiar nerve agents, which include sarin, tabun and soman. Novichok agents work in a similar way, by massively over-stimulating muscles and glands. Treatment for novichok exposure would be the same as for other nerve agents, namely with atropine, diazepam and potentially drugs called oximes. The chemical structures of novichok agents were made public in 2008 by Vil Mirzayanov, a former Russian scientist living in the US, but the structures have never been publicly confirmed. It is thought they can be made in different forms, including as a dust aerosol. The novichoks are known as binary agents because they only become lethal after mixing two otherwise harmless components. According to Mirzayanov, they are 10 to 100 times more toxic than conventional nerve agents. Photograph: Matt Cardy/Getty Images Europe

The Aeroflot flight took off at about 10.30pm. There is no prospect of either of the men ever returning to British soil. Their real identities and current whereabouts are unknown.

Theresa May told the House of Commons on Wednesday that the conspiracy must have been approved at a senior level outside the GRU. This was code for Vladimir Putin, the man whom a public inquiry found in 2016 had “probably” signed off on the operation to kill Litvinenko. The UK security services say a “body of evidence” points to the GRU.

It seems clear that Moscow continues to view Britain as a playground for undercover operations and is relatively insouciant about the consequences, diplomatic and political. The Skripal attack may have misfired. But the message, mingling contempt and arrogance, is there for all to see: we can smite our enemies whenever and wherever we want, and there is nothing you can do about it.