A married dad has been jailed for eight years after a dog walker spotted him acting suspiciously with an underage girl in his car.
The passer-by was so concerned by what he saw while out walking his pet in a secluded area in Sunderland that he alerted the police, which sparked an immediate investigation.
As a result of the tip-off, detectives arrested Syed Uddin and started an in-depth inquiry into his life.
Phone and computer records revealed Uddin, who had a wife and children, had been supplying the teen with drugs, grooming her and had incited her to engage in sexual activity.
Uddin denied wrongdoing but after a trial at Newcastle Crown Court the 35-year-old, of Anderson Street, South Shields, was convicted of meeting a child following sexual grooming, two offences of inciting a child to engage in sexual activity and two of being concerned in the supply of mephedrone.
Mr Recorder Euan Duff sentenced Uddin to eight years behind bars with lifelong registration as a sex offender and sexual harm prevention order restrictions.
The judge said: "The matters came to light thanks to the commendable observations and actions of a gentleman out walking his dog.
"You were discovered in a car with the girl in question and you were arrested, questioned and bailed.
"Undeterred by having been arrested, you maintained contact and continued in the supply of mephedrone to her and incite her to have intercourse with you.
"You exploited her.
"The responsibility of adults towards children is to protect them from exploitation.
"What you did was quite the opposite."
The judge said the dog walker should be commended for his actions and awarded him £250 from public funds.
He added: "But for his actions, these matters might not have come to light."
Tony Cornberg, defending, said Uddin's behaviour was a "fantastical outlet" from his "boring" day to day life but he now wants to become a useful member of society and be back with his wife and children.
Mr Cornberg said Uddin has worked hard since the age of 16 and settled down young after an arranged marriage.
He added: "He describes as lot of anger, directed at himself."