The man behind Ted tells THR that he's been in on one key element of the series finale since the first season: "I thought they weren't going to do that, and then they said, 'No, we're still doing that.'"

The mystery of How I Met Your Mother has always been something of a footnote for the CBS comedy.

Yes, Ted (Josh Radnor)'s 208-episode-long search for a wife is the narrative through line, but up until the eve of the series' ninth and final season, the friendship between the five main characters was always the priority. All that changed with the addition of Cristin Milioti. Identifying the mystery woman, speculating about her fate and wondering how their meeting on that Farhampton train platform will play out drives conversation about the show in the run-up to the March 31 series finale.

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Co-creators Carter Bays and Craig Thomas maintain that they've been clear on the ultimate resolution from the beginning, and it's not a secret they've kept completely to themselves. Radnor, the central player in this puzzle, says he's known part of the finale from the very beginning.

"I was let in on a big thing [in the] first season," Radnor recently told The Hollywood Reporter. "I thought they weren't going to do that, and then they said, 'No, we're still doing that.'"

What "that" might entail is still up for debate for the next week. Milioti recently shrugged off suggestions that the Mother has been dead from the narrator's vantage point since the beginning as "insane," while Bays and Thomas' last public take on the finale was basically a very convivial "no comment."

"We're wildly excited about the finale, and it's great, but obviously we can't say anything," Bays told the crowd at the series' March 15 PaleyFest panel.

Regardless, Radnor says his intel on the ending is not something he ever really focused on. And that bit he was let in on in the first season wasn't even mentioned to him again until they moved into the final run.

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"I knew that one big plot thing early, but it wasn't something that I thought about so much as we were going on," he says. "I always tried to remember that the version of Ted I was playing was moving forward and didn't know everything. He wasn't the narrator [Bob Saget], with the benefit of hindsight. It was always better for me to just read the episodes each week and try to play him as honestly as I could each week. I trusted them that they were going to take him on the right journey."

That journey, nearly a decade long, came to end for the cast when filming officially wrapped on March 1. Radnor, whose last scene was on the platform with Milioti, says he still hasn't fully digested the end of the story -- or his job for the last nine years.

"It was so complicated," Radnor tells THR. "I'm still working through it. It wasn't like I left the set and felt completely emotionally resolved about everything. It was a very complete experience the last two weeks, but I didn't know how to process it actually ending."

How I Met Your Mother's ultimate resolution will play out over one hour next Monday on CBS.