My name's Tom. For the last four years, I've been running the part of MI5, which looks at the threat from the Russian state.

My name is Kate. I've been in the service for about the last 14 years, doing a variety of roles, largely in investigations.

I'm currently working on the Russia mission.

Tom and Kate met me at MI5's little-known underground museum

at their Thames House headquarters in London

to talk about countering the threat from Russia in the grey zone.

It's the first time anyone involved in such top-secret counterintelligence work for the security service has agreed to an interview.

They were using actors to voice their words to protect their identities.

Tom came to the meeting with a small black box in one hand.

He placed it on a table as we said hello and then suggested I open it.

Inside was a smaller pale pink oblong box with the words, Prémyr Jour, Nina Ritchie, Odeparfam,

printed on it in dark purple letters.

It's about the size of a small pack of cards, about half as thick, maybe about the length of my finger.

I flipped it open to reveal a tiny perfume bottle, half filled with a yellowy coloured liquid.

All innocent looking enough, but this is a mock-up of the actual perfume bottle and box

used by two suspected Russian hitmen when they tried to kill former spy Sergei Skripal

and his daughter Yulia with the nerve agent Novichok in March 2018 in the

English city of Salisbury. It is a really powerful demonstration of just how easy it was to

just to slip this into the UK and how reckless the attack in Salisbury was, given it presumably

came in somebody's pocket or some luggage on the flight.

I asked Tom if MI5 could stop another Solisbury-style attack.

He says the agency is doing everything it can, but there are no absolute guarantees in the grey zone.

...