Monger's charge through the field in the rain at Donington Park last April came to a horrific halt when he hit Finnish driver Patrik Pasma's stationary car at 120mph.
'It's still clear in my mind,' says the Surrey-based youngster. 'After we'd stopped moving I could hear Patrik screaming. The adrenaline was running so I felt fine and told the medics to go to him first. It wasn't until five minutes afterwards, when I settled down, that I realised I was in trouble.'
Monger was extracted from the car after 90 minutes before being airlifted to Queen's Medical Centre, Nottingham, where he spent three days in a coma while his legs were amputated.
Perhaps unsurprisingly for a boy who started karting at the age of six, Billy's attention returned to motorsport before he had even woken up.
'I wrote, "Who won the race?" on a bit of paper,' says Monger, who lives in the Surrey village of Charlwood.
'I don't remember doing it because I was in and out of consciousness but my mum showed me the scribble. I can't believe I was asking such silly stuff, but that's what came into my head.
'They saved my life but the accident was unavoidable so there was no point feeling sorry for myself. I just wanted to race again.
'I had messages from all these amazing drivers and they were like a lightbulb going on in my head. I thought, "If you work hard, you can do this".'
Lewis Hamilton was among the well-wishers, with the four-time F1 world champion joining the swathes who raised more than £800,000 for Monger's ongoing treatment and rehabilitation.
The pair are in regular contact after Hamilton invited Monger to the British Grand Prix at Silverstone last July.
'He's been a good friend,' says Monger. 'I know he wants me to do well and is there to support me if I need him. I was star-struck when I first met him because he's my hero. I grew up watching him win world titles and I want to achieve similar things.'
That quest resumed just 11 weeks after Monger's accident when he competed in an exhibition at Brands Hatch.
But a return to single-seaters was out of the question until Monger forced the sport's governing body, the FIA, to overturn a rule which prevented disabled drivers from competing in international series.
Now free to race, Monger returned to the track in Carlin's specially modified F3 car last week.
'I've done some awesome things this year but that was the best,' he says. 'Feeling the speed again and knowing that all the hard work we put in was paying off. There were no nerves. I was going at 140mph but it's the most peaceful I've felt since my accident.
'I'm happiest when I'm in the car. I don't have to worry about my A-Levels or my prosthetics or anything. I can just go out there and be me, doing what I've always wanted to do.'
A trip to F1's jewel in the crown, Monaco, for the Laureus Awards later this month — he is nominated for 'Best Sporting Moment of the Year' for his comeback — will provide perfect motivation ahead of F3's curtain-raiser at Oulton Park over Easter. Not that Monger is lacking appetite.
'I don't want to be a sideshow or just another driver on the grid,' he says. 'I want to fight for wins and show people I can do it properly.'
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