LOUISVILLE, Ky.—The chain of events that led to one of the most shocking upsets in the history of the Kentucky Derby began on Friday morning in Barn 44 at Churchill Downs. A horse named Ethereal Road didn’t seem like himself.

“I thought the last couple of days, Ethereal Road wasn’t as sharp and wasn’t as focused as he was earlier in the week,” said his 86-year-old Hall of Fame trainer, D. Wayne Lukas. “I thought he was a little bit flat. I didn’t think that he was moving as well.”

Lukas scratched Ethereal Road and created the opportunity for one of the greatest understudy performances of all time. His replacement, an alternate, Rich Strike, entered the starting gate here as an 80-1 long shot on Saturday—and then stormed up the rail from far back in a crowded field to win the 148th running of the Derby.

Trained by Eric Reed and ridden by Sonny Leon, Rich Strike outgunned two of the race’s favorites, Epicenter and Zandon, in the homestretch to win America’s most prestigious horse race. Epicenter held on for second, while Zandon finished third.

A $2 win bet on Rich Strike paid $163.60 in the second-biggest upset in Derby history. A 50-cent trifecta wager of the top three finishers paid $7,435.35, while a $1 superfecta of the top four finishers, with 35-1 shot Simplification in fourth, paid $321,500.10.