Irish Mission, a daughter of Giant's Causeway, was ridden to the half-length score by Alex Solis for trainer Mark Frostad and owner Robert Evans. Quaesitor was third, while Seen It All Before was fourth.
Both Irish Mission and Aldous Snow, with Eurico Rosa da Silva up, were near last early in the 1 1/2-mile race through fractions of :25.11, :50.11, 1:14.77, with the mile reached in 1:40.07. Both horses had advanced through the field and were level at the head of the lane after 1 1/4 miles in 2:05.77. The epic duel ended with Irish Mission edging past in 2:30.90 for 1 1/2 miles.
Irish Mission had a stacked resume coming into the Breeders', having scored the Woodbine Oaks, presented by Budweiser, and a runner-up finish to Strait of Dover in the Queens' Plate, the first gem of the Triple Crown.
It's just the beginning for the Irish Mission, bred by Sam-Son Farm, an outfit Frostad trained for from 1994 to 2011, according to the conditioner.
"I don't think you've seen the best of this filly," said Frostad. "I think as a four and five-year-old, she could be any kind of horse."
Aldous Snow, valiant in defeat, is also owned and bred by Sam-Son Farm.
Irish Mission, who ran sixth in the Prince of Wales, got the trip she wanted in the Breeders'.
"She broke right where I wanted her to," Solis said. "I just wanted to be patient. The first quarter of a mile, I let her lope around there and when I got to the backside, I said, 'Stay patient'. At the five-eighths pole, I started picking them up little by little and got in a good position at the five-sixteenths, and just had to wait."
Frostad said Irish Mission didn't like the dirt surface at Fort Erie, but thought the Breeders' turf course was her type of track.
"She didn't handle that surface at all, but she came back and was training well enough that she deserved at shot at the third gem of the Triple Crown," Frostad said. "We knew this was her trip and that she would have no problem with the mile and a half."
No excuses for Aldous Snow, according to da Silva.
"I could not ask for a better trip. The horse was very relaxed and he gave everything he could. He ran big, but the filly never let me go by. I had all kinds of horse (at the top of the stretch). I was not even moving yet, I was just starting to run and I knew he was going to give a good kick. I was surprised she could hold me off."
Trainer Ian Howard managed to guide Quaesitor to a surprise third-place finish in the Breeders', which offered the second biggest field in the Triple Crown era with 14 runners this year (15 ran in 1987).
"I was very happy. He was running on end. He got a little bit outkicked at the head of the lane, but I was happy with him. Those two runners (Irish Mission and Aldous Snow) were very game and she (Irish Mission) obviously likes the grass. I give her a lot of credit."
For the victory, Irish Mission, who is the first filly to win the Breeders' since Sweetest Thing in 2001, collected $300,000 and now has banked $851,671 in her career.
It was a fourth Breeders' stakes tally for Frostad, who also saddled Charlie's Dewan (1995), Chief Bearhart (1996) and Portcullis (2002) to victory. He is now second among trainers in the Triple Crown era behind Roger Attfield (8).
Irish Mission paid $11.80, $6.10, $4.70 combined with Aldous Snow ($5.80, $4.20) for a $54.60 exactor. Quaesitor ($12.10), a 29-1 longshot, rounded out the $1,132.40 triactor. Seen It All Before, at 18-1, was fourth to complete a $4,741.35 superfecta.