Barney urges Ted to sleep with a younger woman he's been dating, but is shocked when it ends up being his half-sister. Robin adjusts to her status as an engaged woman.
This episode provides examples of:
- Age-Gap Romance: Deconstructed, despite mostly being Played for Laughs. Ted has trouble connecting with Carly due to their considerable age difference.
- All Girls Want Bad Boys: Lily starts to go overboard with roleplaying Marshall as a drug addicted musician.
- Callback: To Barney being allowed to marry people, last mentioned at Lily and Marshall's wedding.
- Deliberately Painful Clothing: In one episode, Marshall wears a leather bracelet because it makes him look "tough" to Lily and it turns her on, but throughout the episode, his hand gets more and more swollen because he's allergic to the leather, and by the end it's like a balloon.
- Geeky Turn-On: Ted decides to sleep with Carly after finding out that she likes Star Wars.
- Ironic Echo: Throughout the episode, Barney keeps referencing the Running Gag that he may or may not have banged Ted's mom and winks at Ted every time he says it. Later, when Barney asks that Ted never sleeps with his half-sister again, Ted and Carly wink at him in the same manner.
- It's All About Me: Robin craves attention once she realizes that the engagement ring made her invisible to male attention.
- My Sister Is Off-Limits!: Barney's reaction when he finds out that Ted's Girl of the Week is his half-sister.
- Shotgun Wedding: Barney attempts to marry Ted with Carly after he finds out about their one-night stand.