Gregory had spent two years travelling around Thailand and living in Bangkok, sometimes working as an English language teacher. Suffering a bout of dengue fever, Gregory was unable to work; her money was running out and she was desperate to return to the UK. She met and became friends with Robert Lock from her home country. She claims that soon after they met, he made her a proposal, "Come to Tokyo with me and if you hide my drugs on your person, I will give you £1000." Gregory agreed to carry Lock's 89 grams of heroin and 600 capsules of temazepam, for his personal use, from Bangkok to Tokyo.

On arrival together at Bangkok's Don Muang Airport on February 3, 1993, Lock, Lock's girlfriend, and Gregory were detained after a tip-off about Lock. While security personnel interviewed Lock, Gregory was also observed to be nervous. All three travellers were searched and x-rayed. Gregory was the only person carrying illegal items; the drugs hidden inside her body were detected. Lock and Gregory were arrested; the girlfriend was released. Gregory was held at the Lard Yao Women's section within Bangkok's Klong Prem Central Prison, notorious for its brutality, drug abuse, squalid conditions and severe overcrowding. She befriended other foreign prisoners Karyn Smith from the UK, and Australian Nola Blake.

Gregory spent three years at Lard Yao before being convicted. In court she entered a guilty plea to possession and attempting to smuggle heroin out of Thailand. She was only carrying 89 grams; it is standard procedure in Thailand to give the death sentence to those smuggling more than 90 grams of heroin. Lock pleaded not guilty; he was found not guilty by the courts and released. Gregory was found guilty and was sentenced to death. This was subsequently reduced to life imprisonment, then downgraded further to 25 years the same day.

After spending four years in Lard Yao, Gregory was transferred at her choice to Britain to complete the remainder of her sentence (21 years). She found she was imprisoned in maximum security prisons there: in the UK prisoners were classified by the length of their sentence despite the fact that in the UK trafficking would have attracted a much shorter sentence. In one letter to her parents she said "I wish I had stayed in Thailand". She spent hours confined to her cell in England. In Lard Yao prisoners had the freedom to roam around the prison and the grounds. At least in Lard Yao she was "happy" as she could see the blue sky and the green trees in the prison garden.

After years of long and gruelling campaigning by her parents for her release, the King of Thailand granted Gregory a Royal Pardon and she was released on June 18 2001. She has stated that she is disappointed in her actions as -among other things - it will mean that she will not be able to return to "the country I grew to love."

A few weeks after being released from prison, Gregory applied to Oxford University. She started studying Geography as a mature student at Harris Manchester College[1] in 2002. Prior to attending Oxford, she took a year out to tour schools and talk about her experiences in the hopes of dissuading others from making the same mistakes she did.

"I thought I would be cheeky and I applied to Oxford. I thought, 'When I don't get in here I can apply somewhere else,'" she said. "I didn't think I was going to - but lo and behold, I got in."

"Everyone has preconceived ideas about Oxford and I thought, 'My CV looks blotchy, I've got a criminal record and I'm not part of mainstream society so I'll not get a place.' But I did and I found that the college I had applied to takes people on their individual merits. They accept a diversity of students who have something to offer."

Gregory appeared in the 2006 British documentary series Banged Up Abroad where she tells about the experience.

In December 2009, Gregory attempted to re-enter Thailand, but was refused entry despite having a valid visa. According to friends Gregory wished to visit friends she made in jail and "tie up loose ends."