The episode begins with Barney in old man makeup telling a bimbo that "young Barney Stinson" cannot solve global warming unless she sleeps with him. He then comes back a moment later and hooks up with the bimbo. Soon after, the gang is packing their things for Lily and Marshall's move to the new apartment and Ted's move to New Jersey. Robin states that she does not understand all the gang's sentimentality about their stuff, saying she packed for Japan in just half an hour the other day. Ted and Marshall end up arguing about who should pay the security deposit for the apartment.
Marshall says that Ted could pay it, citing a piece of missing plaster in the wall (it fell off when Ted put his shelf of World Book encyclopaedias on it) and a chip in the ceiling (made when Ted recklessly swung his Renaissance flail). He also counts a piece of damage which he blames on Robin: Robin was very drunk and acting extremely Canadian, speaking with a heavy accent and reenacting great hockey moments in full costume with stick and puck. She tried to make a play through the door, but Lily stopped her and they got into a fight. Marshall broke up the fight just as Barney arrived; he was extremely angry because the Bro Code states to never break up a girlfight. As a result, he punches a hole in the wall.
They then get to a scorch mark over the fireplace. Future Ted first has to explain the story of a friend, Stuart: He was an alcoholic, and his friends held an intervention for him (which Barney ruined by bringing in champagne as it was Stuart's birthday). After this, frequent interventions began to take place at the apartment, such as the intervention for Marshall's Seussian hat, Lily's fake British accent that was a product of watching all the James Bond movies, Robin's tanning addiction, and Barney's excessive use of pyrotechnic magic tricks. The last caused the "INTERVENTION" banner to become scorched as well as the fireplace; interventions ceased after that.
Ted finds in a box a new, unscorched "INTERVENTION" banner; he asks the others why it is there. After a little hedging, they admit they were planning an intervention for his engagement to Stella. Ted demands that they hold it and asks them to read their letters: Lily mixed hers up with a student's, Robin did not write one because she thought, as his ex-girlfriend, she would be biased, and Barney accidentally sets his on fire before getting past the "marriage is the biggest mistake of your life" part. Marshall then reads his letter, which feared that Ted and Stella were rushing into things and did not know each other well enough. He says that he doesn't feel that way anymore and that Stella is "awesome".
After this, Ted ends up getting caught up in a bout of nogistaltia and says he'll never leave the apartment. Marshall and Lily at first oppose him but then quickly jump onto the bandwagon. Robin tries to stop them and then repudiates moving to Japan. They then enthusiastically go down to McLaren's to order the usual.
At McLaren's, Barney is trying to attract young women while in old man makeup to prove to Marshall that he will still be hooking up with 22-year-old bimbos when he is 80. Eventually, he gets the attentions of a French woman. The gang realizes upon viewing his antics that they are trying not to grow up and all decide to move to the places they will move to. They then kick a couple out of their usual booth and swear to each set aside ten dollars a week and, this time next year, have a $2500 bottle of scotch (which they tried to buy earlier in the episode). The show then flashes forward to that time in 2009, where they drink the scotch and decide it is no different from $10 scotch. They then say they will go up to "the apartment" and leave.
At the end, Barney enters the apartment in his old man makeup to see the group with letters and the "INTERVENTION" banner up. They tell him to stop doing the old man thing, at which Barney pretends he is hard of hearing.