Even though Ted has nothing in common with her, Barney begs Ted to continue seeing his much younger, much wilder girlfriend, until he learns who she is. Meanwhile, Robin must adjust to the power of her engagement ring.
While the others are at the bar, Ted walks in and starts telling them how he saved an old lady pinned under a cab. However, the others are paying more attention to the leather wristcuff he is wearing, which they start mocking. Ted tells them that he has a date because of the cuff, but she can't come in the bar, as she is only 20 years old. The others rush outside to see her, but Ted's gets a text and tells them that she had to leave to go to a flash mob. Ted tells them that he met her when he went to a leather cuff shop where she is the saleswoman. Ted later tells the gang after the next date that it was a total disaster. He tells them that he didn't understand half the words she said, they ate snacks consisting of weird food combinations, and she perceived him to to be much older than he really is. Although he thinks she is hot, Ted tells them that he has to end it.
While Ted is in bed, Barney comes to his bedroom and wakes him. He tells him that he has to sleep with his girlfriend. Barney explains that although he is in love with Robin and wants to be with her, his body is detoxing after years of one-night stands and that Ted has to proxy bang his girlfriend for him. However, Ted tells him that he has to connect with a girl at least a little in order for them to hook up. Barney asks him to find something simple to connect with his girlfriend. At their date, Ted tells Barney that he failed to connect with his girlfriend. However, when Ted is about to end thing with her, he hears her R2D2 ringtone, and realizes that she likes Star Wars like him. Later at the bar, Ted tells Barney that he hooked up with his girlfriend. He shows Barney a picture of her and Barney is shocked to see that Ted's girlfriend is his half-sister Carly. He becomes furious at Ted, ends their friendship and storms out of the bar. However, he later calls Ted and apologizes for over-reacting. He forgives Ted and invites him to his apartment to smoke cigars. While Ted is at Barney's apartment, Carly also arrives there, leaving both of them confused. He then arranges a shotgun wedding for them Barney tells him that if they don't get married, this will just be another one-night stand. Ted tells them that they are not getting married. But Barney tells them that they have to, otherwise Ted and Carly sleeping together is another "cheap, meaningless, disgusting" one-night stand. However, Ted realizes that Barney being opposed to one-night stands means that his detox is done and he is over one-night stands. Barney happily announces that he is free.
With Robin's engagement ring getting back from being re-sized, she starts wearing it in public. The next day she tells Marshall and Lily that while she usually gets free stuff, like her morning coffee and bagel, today was different; she had to pay for her stuff. Marshall and Lily tell her that her ring makes her invisible to men. Later at the bar, she has to wait her turn to get drinks (instead of guys letting her cut in front of them, as usual), which annoys her. She momentarily takes off her ring to get her drinks, but becomes invisible to the men at the bar when she wears it again. She tells Marshall and Lily that although she loves Barney, not getting attention by wearing the ring sucks. However, Marshall and Lily tell her that nothing beats the rush you get when "that one special person" looks at you and everyone else becomes invisible. Later that night, Robin understands the true power of her ring when she looks at Barney and everything else becomes invisible when she looks at him. Marshall asks her how she is going to get a drink at the bar now, and she tells him that she knows a way that will last forever. She then asks Barney to get her drink for her.
Meanwhile, when Marshall wears Ted's leather wristcuff, Lily asks him to take it off as he looks like some loser from a band. However she gets turned on by it and they start kissing. Later at the bar, Robin notices that Marshall's wrist has swollen as an allergic reaction to the cuff. She asks him why he doesn't take it off, but he replies that after having a child, anything that gets you close to having sex is worth the pain. Later, Lily notices Marshall's swollen hand and asks him to take the cuff off. Marshall says that he doesn't want to as it will stop they sex they have been having recently, but Lily tells him that he doesn't need a cuff for her to find him sexy. Later, Lily gets a pair of elastic waistband jeans by mistake. Marshall says that she looks like a suburban mom, which he gets turned on by and they start kissing again.
Oh, yeah. I guess it’s a little swollen. I must have strained it lifting.
No, I believe our dear friend Lily was referring to your other wrist. The one wearing the male birth control.
I was gonna go with “chastity bracelet”.
God! Is it possible to love you more?
Do you think your super strength came from wearing Wonder Woman's magic wrist cuff?
I'll have you know that because of this cuff, I have a date.
With some hand lotion and the internet?
It is possible to love you more!
She is hot!
So why can't she come in, Ted?
She can come in! In six and a half months.
Wait... Are you saying she's...
Twenty. And a half. Almost.
(while looking at Robin's engagement ring) Oh, my God! Look at the size of that rock! Barney, you don't start with the "I got caught cheating" diamond. You give yourself room to grow!
It's a family heirloom.
Yeah, I got a few dirty looks from Barney's half-sister over Christmas. So I had to sit her down and explain very respectfully... that she can suck it!
(talking to Robin) Oh, sweetie, I know it's nice to be groped by strangers, but nothing beats the rush you get when that one special person looks at you.
It’s true. Like, here, in this bar, there's only one thing that I see, and I see her brighter and more clearly than anything else in this world. You know what that is? That’s love. And possibly alcohol mixed with some very powerful antibiotics.
And that's Barney sees you. And I know that's how you see Barney.
—Lily and Marshall tell Robin how great life is when you find that one special person.
Now, before you say anything about the hat, this new girl's ass is phenomenal!!
—Ted saying to Lily, Marshall and Robin about the new girl he dates
That night, even though she still couldn't get a beer, Robin realized the true power of the ring.
Hey, Robin. Did you ever figure out how you're gonna get a drink at the bar now that you're engaged?
I know a way that's gonna last forever. Scotch on the rocks?
Coming right up.
—Robin realizes that Barney and her love each other according to Marshall and Lily's theory, after they glow when they see each other.
It's the ring, Robin.
The ring has power.
When a woman puts on an engagement ring, it's like when Bilbo Baggins wears the One Ring in The Hobbit.
Okay, can you say that again but not in nerd?
Sure. Uh, the ring is like the cloak that Harry Potter wears to sneak around Hogwarts.
In Field Trip, Ted's students didn't have problem going in MacLaren's even though they were only 18 year olds. This is inconsistent with Ted claiming that Carly is not allowed in MacLaren's because she is under 21.
While robin orders beer at the bar, she orders 2 beers, but recieves 3
It may have depended on the time. At this time it is night but when he brought his students in it was 4:00 P.M. A lot of bars operate on a rule that after a certain time of night you must be 21 or over to enter.
This episode disproved the theory of Barney's sister being the mother. Carly was first mentioned in Legendaddy. He was told that his sister was in college. This lead to the belief of Carly being the mother.
Donna Bowman of The AV Club gave the episode a C+. She says that Barney’s long history as a character defined by his womanizing has to be a major focus of his post-engagement storyline. But “Ring Up” treats this history either as a source for lazy jokes, or as a story problem to be gotten past as quickly and sloppily as the show can get away with. The script is by Jennifer Hendriks, whose “Bad News” is a series low point for me, but she can’t bear the blame alone; the handling of Barney’s transformation into a one-woman man is something that comes down from a higher creative level. 
Max Nicholson of IGN gave the episode 4.9 out of 10. He says that Ted's arc was "an inappropriate story to tell your teenaged kids". Regarding Barney's arc, he says "there was no detox for Barney's previous engagement, so it seemed a bit odd that this would be a problem now." About Robin's arc, he mentioned that "it was a recycled storyline from Season 1, in which Lily went through the exact same problem following her own engagement to Marshall". However, he also called Robin's musical number "somewhat amusing". Moreover, He also called Marshall and Lily "the couple with nothing to do". In the conclusion of his review, he says that "no single story arc pulled through as particularly engaging or eventful, and there were seldom any laughs" and that this episode "was one of the bigger duds this season". 
Angel Cohn of Television Without Pity gave the episode a B, saying that although the episode was "not terrible, it wasn't fantastically funny either. ...While it didn't further any major plotlines, it at least had a lighthearted sensibility about it." He called the episode "not a total torture". 
The St. Petersburg Comic Review gave this episode 7 out of 10 stars. "...what's up with Ted's wrist band?"