After a massive breakthrough in 2011, they overcame the death of former leader Jack Layton and spent four years developing dozens of new MPs.
Most of them are now gone, along with high-profile veterans such as Megan Leslie, Jack Harris, Paul Dewar, Peter Stoffer, and Pat Martin.
The party is essentially now back to where they were before 2011.
The room at Montreal's convention centre was barely half full, with a few hundred people packed around the podium at the event's peak. The mood was glum.
Party operatives nevertheless tried to spin the results as positive and maintained that leader Tom Mulcair ran a solid, principled campaign.
Mulcair, for his part, said the defeat of Stephen Harper was still a win.
"We've said from the beginning that this election is about change," he told the crowd in French. "And tonight, Canadians turned the page on 10 long years and they rejected the politics of fear and division."
But the results are undeniably poor for the party that entered the election leading in the polls. Despite the results, Mulcair did not resign — and his main spokesperson told BuzzFeed Canada the NDP leader plans to stay on the job.