It turns out that both Lily and Marshall have been writing thank you notes and busy preparing the "Death Folder" - contains statements, documents, info, and letters for either spouse when the other dies.
While Lily is away, Marshall admits that he forgot to write the 'Letter' which leads to...
Meanwhile, at the bar, Robin shares her experience in Argentina with Ted, Barney and Lily, and resolves to be the person she was on vacation. Barney claims she never change and disapproves her relationship with the Argentinean Adonis (this scene is really awesome when they switch to big, complex words so that Robin's boyfriend could not understand).
Ted and Barney set to pose out as out-of-towners to pick up girls after being amazed by Robin's boyfriend on how easy he got girls' attention. So they visit the greatest city on Earth - New York! (Yea...NEW YORK - where they live)
However, their plan gets backfired, somehow.
Marshall and Lily have their FIRST fight as a married couple because of the letter - but they resolve it, somehow.
In the end, Robin breaks up with her boyfriend over certain issues. This episode ends with a scene in the future (Year 2029) - Older Lily and Marshall arguing over the 'Letter', again.
-Images courtesy of CBS.com-