The actress, who plays Robin on the CBS sitcom, told USA Today that her character's relationship with Barney (Neil Patrick Harris) will be key to the episode.
"Robin has had a rough year," she added. "There's been a lot of mostly downs, so I feel like the ending [of the season] is a beginning."
Smulders also revealed that she has enjoyed watching Robin "evolve" since How I Met Your Mother began in 2005.
"It's nice being on for seven years, because you get to evolve," she said. "You get to have babies, and you get to have relationships that fail or prosper.
"You get to see these people grow up and imagine what their lives are like, and you kind of feel part of it."
Smulders will next be seen playing Maria Hill in Joss Whedon's superhero movie The Avengers.
How I Met Your Mother airs on Mondays at 8/7c. The two-part seventh season finale airs on May 14 in the US.
> How I Met Your Mother exec teases 'satisfying' season finale
Watch a promo for How I Met Your Mother below: