Harrington went into the winter with an embarrassment of riches in the three-year-old filly department, seemingly having three top-class inmates.
However, Milisle was beaten in the 1000 Guineas at Newmarket and Albigna fared little better when favourite for the Irish version.
The latest challenge for early-season Group One honours came with Alpine Star (9-2), who was making her seasonal debut and relished the step up to a mile, powering clear in the closing stages.
A half-sister to Alpha Centauri, who won this race herself, Alpine Star was one of the first off the bridle as Run Wild set a strong pace.
When she capitulated, though, Guineas runner-up Cloak Of Spirits took over only to be challenged by Quadrilateral, while Alpine Star shot through a gap on the rail when her stamina kicked in.
American raider Sharing made ground from the rear into second, but Alpine Star was going away by the line, winning comprehensively by four and a half lengths.
Dettori said: “It’s great. I really fancied the filly, but I didn’t want to jinx it. I kept quiet. Jessie approached me about 10 days ago when she knew their jockey couldn’t fly here.
“I knew she was an exciting ride. I asked John (Gosden) permission to ride this filly and he said yes. Everything went to plan. I had a decent draw. I hit a flat spot, but I got a split on the fence and the rest is history.
“I’m delighted for the Niarchos family. Maria (Niarchos) has supported me in the past. I think it was my first ride for Jessica.
“Shane Foley rang me this morning. It has been a team effort.”
Dettori has now won every Group One that is run at Royal Ascot.
He added: “Now I can say I have really conquered Ascot completely now. It’s a big tick in one of my boxes. I’m delighted for all people concerned.”
Harrington said: "We were kind of thinking of going for the French Guineas, but we weren't allowed to go there and then with Albigna going for the Irish Guineas we thought we'd send Alpine Star to Ascot.
"I'm very lucky to have fillies like that. She was amazing. She was having her first run of the year. I was worried all the other fillies had had a run and she hadn't.
"I have no idea (where next). I need to talk to the owners and Alan Cooper the racing manager and talk to Frankie see what he thinks and then we'll make a decision.
"We'll do the best we can for the filly. All I know is the filly likes that little bit of ease in the ground, so I would not ever be running her on good to firm ground."
She added: "She's very laid-back, Alpha was an imposing filly - this filly is not big and not a great colour, you wouldn't pick her out in a ring, but she has a wonderful attitude.
"She doesn't burn up the gallops, she'd never inspire you but every time you give her a squeeze she responds - Frankie was riding some way out.
"You're nervous watching from Ireland - it's unbelievable. It's sad for Shane (Foley) as he would have been on, but we're in strange times. I'm just thrilled to have a winner there, it's something we'll never forget."
Oisin Murphy said of the Graham Motion-trained Sharing: "It was a good run. I was very happy with the way the race went, but she is a champion on really fast turf and she didn't get her conditions.
"Everyone should be very proud and hopefully that will encourage Graham to send more horses next year. She was the best juvenile turf filly in America last year and she is a huge physical filly."
Harry Charlton, assistant to his father, Roger, said of third home Quadrilateral: "She travelled the best, then got to the front and rolled about a bit. Not many horses have been running well off the back of runs this year.
"She travelled good and looked like she would win it nicely, but didn't go through with it in the end for whatever reason. She is a very sound horse, but something might come up. It wasn't to be today."Last chance to get your free month trial of Racing TV! Every race live from over 80 meetings from some of the best British and Irish racecourses. Click here for more details.
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