BOSTON (CBS) – You never know what’s going to happen in the Boston Marathon. Never has that been more true than on April 16, 2018.

It was cold – in the 30s. It was raining – pouring at times. It seemed like every mile another elite runner dropped off the course. It was under those conditions that one of the greatest Boston Marathon stories unfolded.

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“Torrential downpour, 30 degrees, and hypothermic conditions,” Marblehead native Shalane Flanagan recalled in a recent interview with WBZ. “Early on in the race we were running quite slow because the weather conditions were poor.”

“The weather was pretty rotten so I wasn’t incredibly optimistic,” fellow American runner Des Linden told WBZ-TV in the days after the race.

You could see the two elite athletes having a conversation on the course – unusual for American runners in the midst of a race of this magnitude.

“Des let me know that she didn’t feel well and that she was potentially going to be dropping out of the race,” Shalane revealed. “She said, if there is anything I can do to help you, or help the other American women, let me know.”

“I was not too excited about my chances, so I just shifted to – how can I help the people around me,” Des explained.

The pace stayed slow, causing the elite runners to get even colder.

“Any minute I was expecting her to pull off the course,” Shalane said of Des, “but she still hung in it.”

Then in Wellesley, something happened that changed the narrative that might have unfolded that cold April day.

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“I let Des know that I had to go to the bathroom,” Shalane recounted. “I didn’t say ‘wait for me,” but she was like, ‘ok, I’ll see what I can do.”

Shalane Flanagan broke away from the lead pack briefly to hit the port-a-potty during the 2018 Boston Marathon. (WBZ-TV)

Shalane thought her fellow American would surge to the front and slow the lead women. Instead, Des hung back and waited. Thirteen seconds later (“I think I deserve an award for being so quick,” Shalane joked), the pair rejoined the race and caught back up to the lead pack.

“Running can be this incredibly selfish pursuit, so when you have an opportunity to help others, those are the moments that are worth it,” Des said. “In that process of helping someone else, I really found my rhythm from there.”

That bathroom break truly changed the race.

“I think that moment kind of rejuvenated her,” Shalane said. “She actually ended up feeling better having to sprint a little bit to catch back up; it warmed her up I think.

“She was in this act of selflessness. It’s amazing because it ended up really helping her.”

The lead pack started to disintegrate. Des moved to the front. The race she had earlier written off was now hers to win. And she did. Des Linden became the first American woman to win the Boston Marathon in 33 years.

“We were so desperate for an American to win,” Shalane said, looking back. “We really acted as a team out there and that communication, that familiarity I think it helped us.”

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That win, in those conditions, with that selfless act, is now another amazing chapter in the story that is the Boston Marathon.