Super Ringer

Replacing a player in a contest with an absurdly high-ranking replacement pulled in from nowhere—a Super Ringer.

A typical plot using the trope would play out as follows: Alice is our plucky heroine. Elena is her Latina best friend. Suzie is the Alpha Bitch and Tina is The Dragon in Suzie's Girl Posse. The game is doubles tennis, it's the final and the prize is $1,000 to the charity of the winners' choice. Naturally, Suzie will use it for nefarious ends.

Near the end of the episode, something happens to Elena; she breaks her arm after slipping on a wet floor. She can't play. Surely the evil Suzie cannot be stopped. Alice has an idea. On the day of the game, she comes along with another player.

"Hi, everyone", she smiles sweetly, "this is Maria".

Suzie's mouth drops open and she promptly concedes. Never mind that you can't just randomly change a player who gets injured during an event, you would have to forfeit.

Compare and contrast with Put Me In, Coach, where a previously-underestimated player shows her talents in the final stretch. A common subversion is for the Super Ringer to get injured or incapacitated forcing the team to rely on the underestimated player.

Examples of Super Ringer include:

Film - Live Action

In M*A*S*H, they bring in Spearchucker Jones (who played professional football with the San Francisco 49'ers prior to the war as well as being a surgeon). He is transferred to the 4077th to help them win a football game against a rival outfit. Then the other team starts winning anyway. How? Jones notices someone on the other team -- or, in Radar's words, 'Our ringer spotted their ringer.'
In the Abbott and Costello movie Here Come the Co-Eds, a professional women's basketball team is secretly brought in to play a women's college team. (There's heavy betting involved. The college's future is at stake.) One of the college players is injured, so Lou Costello puts on a dress and a wig and goes in as a sub. He's terrible, but after he's knocked out mid-game, he awakens with amnesia and is told, "You're Dolly Dimple, the world's greatest woman basketball player!" Living up to his billing, his team wins. At the end of the game, all of the ringers are revealed. The officials decide that "Five ringers are worse than one" and award the game to the college players (and Lou, who isn't in college.)

Live-Action TV

An episode of Kenan and Kel had the titular duo bring on the NBA player who'd been the Special Guest Star as their third player in a game of basketball against some people who challenged them earlier. Their opponents defaulted.
The West Wing: In "The Crackpots and These Women," a variation of this trope occurs, in which President Bartlet has recruited a ringer (a former Duke Basketball player and Final Four contender) to help him beat the rest of his staff at a game of basketball. A slight aversion because there's no indication given of who would normally be playing or why they aren't.
Family Matters, Extroverted Nerd Steve Urkel tries to recruit Jerk Jock Eddie Winslow as his partner in a 2-on-2 inner city basketball tournament. Despite showing some very respectable ball handling skills, Eddie refuses and goes with someone else. Undaunted, Steve shows up at the tournament with a tall, muscular woman in a dress and bonnet whom he only calls "Grandmama". Sure enough, the two teams face each other in the finals. But defying trope, Grandmama is injured during the game, so Eddie's partner steps out and Eddie and Steve play each other 1-on-1 for the last point, where Steve jukes right past Eddie for the winning layup.
In the Numb3rs episode "12:01 A.M.", the CalSci basketball team, which hasn't won in years, is being coached by Prof. Fleinhart, who has a serious problem with losing. He brings in two NBA players as ringers, promising them a ride on the space shuttle. They get a ride on the shuttle simulator in a later episode.

Western Animation

The Simpsons:
- The entire plot of "Homer At The Bat". We're talkin' softball, from Maine to San Diego, talkin' softball, Mattingly and Canseco, talkin' Homeeer... Ozzie and the Strawb". By the end of the episode, Hilarity has Ensued and all but 1 of the ringers are unable to play for various reasons (notably, Mr. Burns fires Mattingly for not getting rid of his sideburns, even after Mattingly shaves the sides of his head).

Mattingly: Still like him better than Steinbrenner.

- Also, in another episode, Homer replaces Lisa with Serena Williams as his tennis partner. Then Marge replaces Bart with Venus Williams. Then Venus replaces Marge with Pete Sampras. Then Homer gets replaced by Andre Agassi. So the family gets to watch a world-class doubles match for free.
In the Futurama episode Time Keeps on Slipping, the protagonists are challenged to a game of basketball against the Harlem Globetrotters, so the Professor creates some ringers: a team of atomic supermen.
In Static Shock, Shenice is introduced as the Super Ringer on the girls' bowling team (it's legit in this case since it was a school team and she had just joined the school) where she proceeds to cream the boys until her parents show up and she gutters the last ball. Turns out Shenice is a literal Super Ringer, who also moonlights as the heroine Shebang.