Barney convinces Ted to sleep with a 20 year old until Barney learns her identity, Robin discovers that her engagement ring makes her invisible, and a leather cuff improves Marshall and Lily's love life.
Robin's past as a Canadian pop star is revisited when Barney uncovers the lost episode of a music documentary series in which she was featured as Robin Sparkles. Marshall and Lily have some concerns about Ted's latest girlfriend.
It's weeks before Barney and Robin's wedding, and one of the many things they still have to decide is where to live. The one thing Robin insists is that they don't live at Barney's apartment. So it goes on the market. However, Barney doesn't want to sell it to just anyone, but someone who is worthy of what was and will always be in his heart his "Fortress of Barnitude". Robin has a slightly different idea, which isn't as easy to achieve as she would like when she and others learn more and more about with what Barney has equipped his apartment. Meanwhile, Lily hasn't ...
Lily meets an interesting person on the train to Farhampton, Marshall gets kicked off his flight to New York, Barney and Robin discover that they might be related, and Ted prepares to deliver a personal gift to Robin.
Barney and Robin try to have sex before the elderly wedding guests arrive, Ted reveals a list to Lily of things to do before he leaves New York, and Marshall has problems traveling through Packer Country.
When Ted finds himself with three prospects to be his date for the wedding weekend, he chooses poorly. Meanwhile, Barney and Robin have a confrontation with their minister, and Marshall learns more about his driving companion, Daphne.
When Robin and Loretta's conflict escalates, Barney is caught in the middle. Meanwhile, Marshall and Daphne deal with a stowaway on their road trip, and Ted and Cassie try to enjoy a trip to a lighthouse.
After Robin finds out her mother isn't coming to the wedding, the group recalls a fateful night six months ago when Barney accepted the "challenge" of a lifetime, and Ted made a decision about his pursuit of Robin.