Clean Up the Town

A new person (or an old inhabitant returning after years away) comes to a town suffering from lawlessness and corruption. He or she is appointed to a position of responsibility, such as The Sheriff, and proceeds to reduce crime, establish the rule of law, and topple the corrupt powers that kept the town from prospering.

A stock plot in The Western, although easily transplanted to other settings. Often, it may be a corrupt organization that needs cleaning up instead.

In more cynical works, the would-be reformer ends up succumbing to the corrupt system, dying or worse, becoming just as corrupt as the people they replaced.

In comedic works (particularly cartoons, it seems), may entail a broom and/or dustpan, a pointy stick and garbage can, or other literal interpretations of the phrase.

Contrast with the Tyrant Takes the Helm story arc, where the villain may believe that they're in this plotline, but are actually making things worse.

Examples of Clean Up the Town include:

Comic Books

Batman, to varying success; the recent franchise reboot emphasized this aspect, as Batman himself hopes that someday he won't be needed.
Jesse Custer in the Salvation arc of Preacher (Comic Book).

Film

RoboCop
Example of comedic use: Back to The Future:

Goldie Wilson: You wait and see, Mr. Carruthers. I will be mayor! I'll be the most powerful man in Hill Valley. And I'm gonna clean up this town.
Lou: Good. You can start by sweeping the floor. [hands Goldie a broom]

Jimmy Cagney does this with, of all things, the Bureau of Weights and Measures in Great Guy.
Played straight to hilarious effect in Blazing Saddles when the new sheriff has to clean up the town and resorts to unorthodox methods. Candy Gram, anyone?
In Hot Fuzz, Nicholas Angel has been so effective at cleaning up London that he's making the other cops look bad, so he's transferred to the quiet town of Sandford. At first it looks like there's nothing for him to clean up, but then unusual deaths start happening...
- And when all the crime stats back in London go off the charts in his absence (making the Police look even WORSE than they did when Angel was just showing them all up), they come crawling back to beg him to return.
Road House. Dalton starts off acting as head bouncer at a bar, but is forced to Clean Up the Town to save his own life.
Happens in (of all places) Sgt Peppers Lonely Hearts Club Band, after Mean Mr. Mustard's theft of the magical musical instruments turns the town of Heartland into a crime-ridden cesspool. How does the title band clean things up? With a musical carnival!
This is a good deal of the plot of Walking Tall.
Outland
Dodge City

Literature

Carrot Ironfoundersson, when he arrives in Ankh-Morpork - but he's so nice about it...
Pat Rin yos'Phelium when he arrives on Surebleak.
Deconstructed in Harlan Ellison's short story, "The End of the Time of Leinard". The sheriff who was brought in to clean up the town decades before is now seen as a menace for his heavy-handed tactics, leading the town to conspire to get rid of him.

Newspaper Comics

Parodied in The Far Side: a janitor is praised for "cleaning up the town" as he walks into the sunset.

Tabletop Games

Shadowrun adventure Harlequin's Back. In one of the mini-adventures in the book, "A Fistful of Karma", the PC's must defeat a cruel tyrant who oppresses the people of a mining town.
Dragon magazine #71 (back when it wasn't just Dungeons & Dragons) had a Boot Hill module called "The Taming of Brimstone", in which the player characters had to clean up the eponymous town.

Live Action TV

The Untouchables.
Many episodes of Stargate SG-1. Often, the SG 1 team arrives on a planet to discover that an enemy force ("Goa'uld" or otherwise) has enslaved or is otherwise tormenting the local population, prompting the team to clean up the town.
The entire cast of Angel was eventually assigned to run the uber-evil law firm they had spent the last four years fighting. Everyone angsted a lot about whether they were up to the task, or would be corrupted themselves.

Angel: You hired Harmony as my secretary??
Wesley: I thought we could use a familiar face.
Angel: Hmm. You turned evil a lot faster than I thought...

Video Games

Grand Theft Auto: San Andreas starts off and ends up being about CJ and Sweet cleaning up their crack-ridden 'hood.
The Nameless Mod's Trestkon is this- if the player wills it.
In Crackdown, the city's police force barely is able to hold the line at their own headquarters. The Agent must go through the city, killing the leaders of the three gangs who rule the city and their lieutenants. It becomes a subversion when it's revealed the Agency allowed the gangs to run roughshod, so they'd be able to assert a despotic regime once they clean out the gangs.

Western Animation

Looney Tunes short "Drip-Along Daffy": Daffy comes to "clean up this one-horse town", and in the end he does... as streetsweeper. "Lucky for him, it is a one-horse town."
The SpongeBob SquarePants episode 'Pest Of The West' has Spongebuck immediately being appointed sheriff so he could save the town from Plankton's ancestor, Dead Eye Plankton. He does exactly that.

Real Life

Name a political challenger, this is almost guaranteed to be one of their platforms.
City Marshal Dallas Stoudenmire was brought into El Paso to clean it up as the previous 5 marshals in the last 8 months were either incompetent, corrupt, or killed. He proceded to kill at least 10 criminals in the next year, dropping the crime rate significantly.

Music

The first album by The Protomen largely takes place some time after Dr. Light created Protoman to do this, and Mega Man decides to follow in his footsteps. Mega Man ends up discovering that Protoman grew so disgusted with the inhabitants of The City and their unwillingness to fight for themselves against Wily's tyranny that he turned on them and became Wily's Dragon.