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After about an hour of chasing, and with the help of some other bystanders, he managed to get the dog back safely home to a delighted owner, having performed a wonderful - if slightly unusual - act of kindness.
It all started because Reuben McCartney decided to get back into riding his bike after the lockdown forced the gyms to close.
While out and about near to his London home, he spotted the pooch - a one-year-old rescue dog called Kiki - in danger and, fearing that she would be knocked over by a car, he started chasing.
Their chase, as you can see from the cycling route map, took them from near to Old Street, via Kings Cross - a few times - and eventually back to her rightful owner.
Luckily, there was a number on Kiki's collar that he could call to get in touch.
Reuben told LADbible: "As soon as I saw that the dog was running with a lead attached to her dragging along on the floor I knew I had to try and catch her.
"My biggest fear was that she'd get run over as she was crossing busy main roads at speed and on a couple of occasions the traffic had to stop or swerve."
He continued: "I chased her for about an hour. My biggest fear that I wouldn't be able to catch up and I'd lose her, which I did a couple of times, but managed find her again.
"My family has always had dogs and they are part of the family, so I'd hope that if that happened to ours someone would do the same."
Now Central London is no place to lose a dog, or to be haring around after one, but luckily this story has a happy ending.
Reuben said he was 'shattered, but it was worth it' after his ordeal. He explained: "The little thing was relentless and every time I got close, she would manage to nip past me."
When he eventually reunited Kiki with her owner, Christina, Reuben said she was 'over the moon' after searching the streets for her missing pet.
For Christina's part, she said: "We were heading out for a walk when a firework went off nearby and Kiki bolted.
"Her lead slipped out of my hand, and because she's a collie, she's a little fast one.
"I got out onto the main street and everyone was pointing towards the direction that she was running, and she basically kept running in the same direction towards Kings Cross.
"Later, after some running and heavy breathing I got a call from Reuben saying that he'd caught her in the canals near Kings Cross and he agreed to walk back to Angel to meet me there.
"I was just a distraught dog mum worried about an accident on the roads or more fireworks going off."
She added: "I cannot thank Reuben enough for going through that effort of catching her, because we live in quite a busy central area with a lot of cars.
"I'm so thankful he got her before anything worse happened!"
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