The Greatest Moment in 'Biggest Loser' History (VIDEO)

The theme for this season's 'Biggest Loser' is "paying it forward." This concept means doing a favor, without expecting anything in return. It's based on the idea that we receive generosity from strangers all the time. Since we can't pay them back, we should move "forward" and do good deeds for others.

Tonight, that theme really paid off. By the end of this episode (Tue., 8PM ET on NBC), there were hugs, tears -- and one man's life changed for the better.

It began when the contestants faced the latest challenge, which involved "Angels Flight," the world's shortest railroad. However, for some players, it proved to be quite a long trip.

Angels Flight is a quirky old railway line. It runs only 298 feet, traveling up a very steep hill in Los Angeles. For tonight's challenge, Ali Sweeney gave the 'Losers' a choice: To climb the hill, they could take the train ... or the stairs. The first player to get 100 points in the game would win a brand new Ford Edge. Climbing the stairs equaled five points; the train ride equaled only a single point.

All the contestants opted for the stairs -- but it would take twenty trips to win the car. Ada and Brendan took a quick lead in the race, while many 'Losers' struggled -- or simply quit. But even though he was lagging far behind in third place, 28-year-old Patrick House refused to quit. Patrick already faces an uphill battle in his life: He's pre-diabetic, and is the unemployed father of two young boys. But even though Ada and Brendan were way ahead, Patrick kept jogging up those stairs.

And then he lost.

... Or did he? Ada came in first. But instead of hitting the red button that would end the game, she paused and waited at the top of the stairs. Brendan arrived second, and he didn't press the button either. Instead -- in a random act of kindness -- they decided Patrick deserved the prize more. When Mr. House chugged into third place, he slammed down the button with both hands -- and won the car. Patrick wept and hugged Ada with gratitude. Fellow contestant Frado called it "the greatest moment" in the show's history. And we'd be hard-pressed to disagree.