Speciosa made all the running to win the 1,000 Guineas at Newmarket.
Michael Fenton rode Speciosa to victory
The filly was bought for just 30,000 Guineas (£31,500) but proved too good for the big guns, beating Confidential Lady into second with Nasheej third.
Speciosa is trained by Pam Sly at her 20-horse yard on the family farm in the Fens and first announced her Guineas potential by winning the Nell Gwyn.
It still looked like too big a task but she got stronger as the race went on and came home a 10-1 winner.
Speciosa set a sensible early pace to head a group on the stands side, while Donna Blini led a group of four racing down the middle of the course.
Jockey Michael Fenton always had the filly travelling sweetly on the rail and she was still swinging along in front as her rivals began to toil hitting the two-furlong marker.
Although Confidential Lady tried to make a race of it, Speciosa just had too many guns and she powered out of the dip to win by two and a half lengths.
Confidential Lady, a first Classic runner for Sir Mark Prescott, held Nasheej by a length for the runner-up spot.
Sly revealed she had turned down an offer of $1.1m (£600,000) for the filly after she won the Rockfel Stakes at Newmarket last autumn.
"It has been well documented that we didn't want to sell - and we won't, not just yet," she said.
"I'm just overawed. It's amazing. I'd hoped she'd be in the first four - it's wonderful."
The crowd surrounding the winner's enclosure after the race was swelled by two coachloads of supporters from Sly's local area around the Cambridgeshire village of Thorney.
"It's fantastic for racing. It just shows that a small little band of people like us - my girls, the jockey, everybody - it's good for them."
Stan James 2,000 Guineas, 1m, Newmarket
1 Speciosa M Fenton 10-1
2 Confidential Lady S Sanders 12-1
3 Nasheej R L Moore 16-1
13 ran. 2½l, 1l.