Not many teenagers from working- class Birmingham backgrounds with new passports, travel on business- class tickets routed from Amsterdam to Bangkok, and plan to return to London via Lagos with more than 30kg of heroin worth pounds 4m.
Neither are there many drug barons who would pin so much faith on two such innocents abroad.
During their stay in Thailand in July 1990, the pair allowed themselves to be escorted up country to Chang Mai - part of the notorious drug-producing Golden Triangle. At the time Smith was 18, and Cahill, 17, but Cahill was the dominant partner and paid the bills.
Cahill, whose passport had been issued in June, had been beaten up in Birmingham when she had tried to back out of the trip. Smith, whose passport was issued just before departure, was very much a last-minute travelling companion.
In Thailand, the two could not have done more to advertise their presence to the authorities, lending weight to the suspicion they were decoys, sitting ducks, sent out to distract attention from professional smugglers.
They missed a flight that left Bangkok on 17 July 1990. They had a booking and went to the airport - but missed the flight because of a confusion over 24-hour clock timings. Such was their sophistication.
But the mishap raised a critical question: Were the real couriers on that flight, and who were British Customs tracking - the big game or the small fry?
As it was, the two women returned from the airport to Bangkok, and checked in to room 8069 at the Grace Hotel, one of the best in town.
The next day, 18 July, the police set up a surveillance operation after an informer told them 'two foreign women were preparing to traffic a large amount of heroin from the kingdom'.
At about 8.30pm Cahill and Smith left the hotel by taxi for Bangkok International airport with two large suitcases, and four other pieces of hand luggage. Before they were checked in, the police pounced on them on the pretext they had found a crisp bag containing heroin. Both women were taken to the airport's Customs Inspection Office.
Karyn Smith's bag contained seven cans of drink and seven bottles of liquid soap and shampoo 'mingled with clothes', it was later alleged in court. Patricia Cahill's bag contained nine tins of sweets and 'drink powder' and five shampoo containers.
The media had been were tipped off and were waiting for the kill. They took photographs and filmed the two women before the alleged drugs haul had even been discovered.
After being cut open, 21 of the containers were alleged to have concealed 21 bags of 'white powder' weighing more than 30 kilos. 'Said white powder, was treated with a liquid to test for narcotics and it turned out to be heroin,' the court was told later.
No independent witnesses saw the discovery of the drugs. Neither the drugs nor the containers were produced in court, even though it was alleged 'precision-made plastic cylinders' had been welded inside them. Although no transcripts of the trial proceedings have been made available, it appears that evidence was given by the arresting officers that Smith denied all knowledge of drugs, while Cahill admitted an offence.
Smith said in an affidavit last year: 'As to knowing I was carrying heroin, the advice I got from my lawyers was that it did not matter that I did not know, which I did not in fact; it was enough that the heroin was in my case and I would suffer severe punishment, even death, if I pleaded not guilty. I was not advised about planted heroin, it was never mentioned . . .'
'When we were arrested and the cases opened, I was shown a white packet of powder which I was told was heroin. I saw no other heroin at any time taken from the cases.' Stephen Jakobi, a London solicitor who has run a tenacious campaign to free Karyn Smith, said her parents were advised - wrongly - that the senior trial judge had passed the message that unless she pleaded guilty she would be sentenced to death, irrespective of knowledge.
'This statement was totally and deliberately false, but its effect, reinforced by the public demand of the prosecutor for the death penalty . . . amounted to torture. They acquiesced after a week,' he said. Visiting Bangkok last year, he learnt that absence of criminal intent was a defence under Thai law.
Smith pleaded 'guilty but ignorant' - not so much a plea as a submission in mitigation of a serious, admitted crime. At the trial in December 1990, she was jailed for 25 years.
Eric Smith, Karyn's father, has fought for her release since her arrest. He said last night he was unsurprised by the new evidence. 'I felt it from the very beginning. The police showed me three containers and told me this is what the heroin was in. I knew very well they could not have held the amount of heroin they said was in there. They couldn't hold anything more than a half- to one-ounce tobacco packet.'
He never doubted her innocence despite the strain the fight imposed on his family. 'It has been a struggle. In every statement Karyn made she said she didn't know anything about it and we believe this is the truth.'
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