On 4 March 2018, former Russian spy Sergei Skripal and his daughter Yulia were poisoned.
Senior defence scientist Professor Tim Atkins took the call from Salisbury Police requesting emergency advice after Mr Skripal and his daughter were found slumped on a public bench.
In total, six people were poisoned during the incident and one of them, Dawn Sturgess, died as a result of exposure to the discarded nerve agent.
It took military specialists 13,000 hours to clear the threat.
Three years after what turned out to be a Novichok poisoning, Prof Atkins from Defence Science and Technology Laboratory (Dstl), spoke to Forces News: "The indicators for me were from a variety of sources.
"They were from the footage that I saw of the individuals, they were from the information clinicians gave me about what was working and what wasn't in terms of treatment.
"There was the background knowledge that Sergei has been involved in espionage," he said.
Prof Atkins said he could feel something was not quite right and "suspected there was a toxic chemical involved".
"I certainly didn't anticipate that it would be a Novichok."
Novichok was developed by the Soviet Union in the 1970s and 1980s.
The substance is designed to counter NATO detection systems for use on the battlefield or in targeted assassination attempts.
The poison is absorbed through the skin and attacks the nervous system.
As the nerve agent takes hold, the body experiences uncontrollable spasms, breathing stops and the heart goes into cardiac arrest.
When Dstl staff confirmed Novichok had been used against the Skripals, it was a game changer.
Prof Atkins said: "I think firstly I felt shock and real disbelief, my blood went cold."
"I needed some time to compose myself and then I think my mind turned to how I'm going to explain this to the people who have to respond to this incident.”
Dstl is the only organisation in the United Kingdom allowed to hold small amounts of nerve agents on site.
"I think the use of something like that is hugely significant," Prof Atkins told Forces News.
"The use of a chemical [like] that within a city where once you release it it's very difficult to control where it goes and what happens subsequently.
"[It has] the potential to affect other people and reasonably large numbers of other people," he explained.
An attack of this kind was unprecedented on British soil and Dstl staff had to work around the clock to combat the deadly compound.
"We had to learn a great deal," Prof Atkins said.
"The existence of Novichok was published, so we knew about the material, but there's no described data on what happens to a person who’s exposed to Novichok.
"It's a continual learning curve."
Three years after the attack on Sergei and Yulia Skripal, Prof Atkins said he looks back at those days in 2018 with both a sense of pride and concern.
"Across those two incidents Dawn Sturgess was killed and that's not the first time my job has taken me into areas of response where individuals [have] been killed and that always stays with you."
"I came out a slightly different person to how I went in," he added.
The Defence Science and Technology Laboratory is now helping the Government respond to the coronavirus pandemic but Dstl remains at the heart of Britain's defence against any potential adversary who may seek to launch another chemical, nuclear or biological attack on UK soil.
Cover image: A library image showing military personnel during the recovery operation following the Novichok attack in 2018 (Picture: MOD).