Trama

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.

Although on the surface they are both happy about their engagement, Robin and Barney contemplate sabotaging that happiness for different reasons. Wearing an engagement ring, Robin eventually comes to the realization that the ring is making her invisible to men, and as such they don't lavish her with gifts as they used to do. She has to decide whether the ring or the free gifts and attention are more important. And although Barney says he is disgusted by Ted dating twenty year old Carly with who he only has a mutual physical attraction in common (she who is into "old" guys), Barney needs Ted to sleep with her if only to live through Ted vicariously. Ted, however, is thinking of breaking up with Carly without ever having slept with her unless he can find something, anything, as a common emotional ground between the two of them. Barney's need for Ted to do this act may be affected by beautiful Carly herself. And Marshall is sporting a new look as it is turning Lily's crank, but that look has an unexpected negative side effect. Marshall has to decide if the side effect is worth the passionate sex with Lily.—Huggo

Ted meets a new girlfriend who turns out to be a leather cuff salesperson just slightly younger than 21 who convinced him to buy a leather cuff (she was the saleswoman for the leather cuffs at the retail store), so the gang is unable to meet her in person at MacLaren's. The girlfriend uses roller blades as a means of transportation across the city and even participates in non-sense protests to buttress her activist credentials.

While he finds her really attractive, their age difference means he has trouble connecting with her. The girl uses weird social networking sites for communications, hangs out at neighborhoods that Ted has never heard of, and subsists on a diet of pointlessly weird food combinations (Kimchi cupcake with a bacon frosting). They head to a club in Chinatown beneath a manhole cover. She notices the white hair in Ted's eyebrows and believes that he fought in the Vietnam war. When he is about to end things with her, Barney convinces him to give it a shot, which will allow Barney to live vicariously through Ted now that Barney is engaged to Robin. Barney begs Ted to have sex with the 20-year-old on his behalf. "Be my stand in and put your gland in" is how Barney phrases it. Ted says that to have sex with a girl, he needs to connect with her on at least something. Ted promises to try to Barney's sake. She doesn't like TV or Sports or Books.

Ted is about to end it with her upon realizing she has a really strong fetish for old people (Her believing him to be a senior citizen despite him only being in his 30s), but finally gives in when he realizes she likes Star Wars.

After they sleep together, however, Barney finds out that she is actually Carly (Ashley Benson), his half-sister, and is furious at this betrayal. Later, Barney attempts to marry Ted and Carly, and mentions that he finds the idea of a one-night stand "cheap, meaningless and disgusting". Both Ted and Barney then realize that Barney is done with his detox from one-night stands. Though happy with this freedom, Barney urges Ted and Carly not to sleep with each other again.

While Barney struggles with his detox, Robin is going through her own struggles as well-she has difficulty getting used to being engaged. Robin now sports a large diamond on her engagement ring, which is very visible even from a distance. Where she used to get freebies by being a single, beautiful young woman (the lines would part for her like the sea and the "stuff" is always free), the ring now deprives her of them-including buying herself a beer at MacLaren's. However, Marshall and Lily assure her that when she looks at the man she loves, everyone else becomes invisible. Later that night, Robin understands the true power of her ring when she looks at Barney, and cleverly solves her problems by asking Barney to help buy her a drink instead.

Meanwhile, Marshall takes to wearing Ted's leather cuff as it turns Lily on. Marshall pretends to be a loser musician, with a Vicodin addiction and a beat-up van that he uses to steal copper wire. This is in spite of the fact that his wrist starts swelling and eventually gives off a horrible smell. Lily finally explains that he does not need the leather cuff to turn her on. Afterwards, Lily receives a pair of elastic waist jeans in the mail by mistake, which turns out to be a big turn-on for Marshall as Lily looks like his mother now.