A simple form of The Reveal used to explain why Police Are Useless, and why our otherwise normal characters don't simply ask the police to deal with the dangerous criminals. It turns out the police are the criminals.

Generally speaking this trope is intended to rationalize why the main characters don't go to the police with their problems, which tends to be the logical response by normal people to outrageous things like murder plots. This can also be established in the backstory and does not need to be displayed on-screen directly.

Often involves at least one Dirty Cop by necessity. Can result in a Have You Told Anyone Else? from the bad cops if someone comes to them in search of a Hope Spot.

Examples:

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Anime/Manga

Fairly early on in Naoki Urasawa's Monster, Tenma runs into a couple of cops who work with/for Johan 'The Monster' Liebert. He doesn't know who else might be, and thus gets really paranoid about talking to the police... rightly so, since shortly afterwards, he gets framed for murder.
Naoki Urasawa's 20th Century Boys features this as a Hope Spot: the young detective hero has successfully contacted an influential ally in the upper echelon of the police force with his information on The Conspiracy. Surely things will turn better from there on right? Wrong - the police is already in the bad guys' pocket and you have just doomed everyone by revealing your hiding place.

Comedy Series

Parodied in a Monty Python's Flying Circus sketch. A pimp is telling an interviewer about his dealings with the Piranha Brothers.
Luigi: One night Dinsdale walked in with a couple of big lads, one of whom was carrying a tactical nuclear missile. They said I'd bought one of their fruit machines and would I pay for it.
Interviewer: How much did they want?
Luigi: Three quarters of a million pounds. Then they went out.
Interviewer: Why didn't you call the police?
Luigi: Well, I had noticed that the lad with the thermonuclear device was the Chief Constable for the area.

Comic Books

A major theme in Batman: Year One is that most of the police department is corrupt — this is given as an explicit reason why it's necessary for Bruce Wayne to don a bat costume and terrorize criminals, as often some of the people he beats up happen to be cops who are in on the crime. The first real progress he makes is when he finds out that Gordon can be trusted, creating a legitimate outlet to fight crime.
In Sin City, the police force is so corrupt, it's actually shocking when people find a cop who is clean. In the first story, police death squads are sent after Marv in order to silence him after he's framed for murder. In That Yellow Bastard almost the entire police force are willing to protect a pedophile Serial Killer son of a senator, going so far as to frame one of their own (probably the only good cop of the entire bunch). In The Big Fat Kill, a recently killed Domestic Abuser is exposed as being a hero cop, which is bad news for Old Town (because the truce between the girls and the cops forbids the girls to kill cops that wander into their territory, in exchange for the girls being allowed to protect their own and keep the cops and the mob out). In Hell And Back, the cops are in league with an assassin guild. At least in Hell and Back the police chief briefly rediscovers his morals long enough to help the protagonist finish off the assassins/human traffickers.

Film

Fletch. Police Chief Karlin and some of his officers are running the local drug ring, so Fletch can't go to them for help.
In Date Night, this is the reason why Phil and Claire end up ditching the police station mid-interview to conduct their own investigation — they see their kidnappers walking in and getting some coffee, and are not at all sure they're safe being around any longer. It later becomes relevant information that the whole police force isn't in on it as the plot is resolved when Holbrook contacts someone he knows he can trust to organize the final operation.
Pineapple Express has this when Dale sees that a police officer in uniform is present when Ted murders a rival criminal — and puts some bullets in him herself just to be sure. Since Dale's a process server, he really should have someone he can talk to about this, but on the other hand, his chronically being high doesn't do wonders for his judgment. Hilariously, he's arrested later on — and the beat officer actually believes his story because it explains iffy problems in the department. Saul completely screws this up by helping Dale escape.
The bad guys in the remake of Assault on Precinct 13.
Hot Fuzz - the Chief of Police turns out to be in on the bizarre "accidents" surrounding the town.
- In fact he's the ringleader; the whole shenanigan is built around winning a contest that's more or less meaningless beyond his wife's annual involvement in the town's participation therein.
The 2008 French movie Skate or Die.
In Fight Club the Narrator tries going to the cops when he finds out that Fight Club (now called Project Mayhem) is involved in serious terrorist activities. Except it turns out several police officers are members of Project Mayhem, and have orders to castrate anyone who betrays the group (even if that someone is the leader of Project Mayhem). Cue the Narrator trying to take matters into his own hands.
Cellular
L.A. Confidential: Captain Dudley and a large group of his men are setting themselves up as the new LA drug kingpins after Mickey Cohen goes to prison.
Hobo With a Shotgun yeah okay they're not really the bad guys, but very, very few of them aren't corrupt.
Chief of Police: "We're ALL bad cops!" to a nother cop.
In West Side Story, Lt. Schrank and Officer Krupke are not only meddling in the affairs of the Jets and the Sharks, but they're also very racist against the Sharks.
Flightplan: The plot was claimed to a Xanatos Roulette, but most don't want to accept that everything happened - and could happen in Real Life because, "People believe what (cops) tell them to believe. That's how authority works."
The 2007 Transformers film does it with a twist - the Decepticon Barricade has a police car as his alternate mode.

Literature

In line with his Anvilicious Libertarian philosophical leanings, Dean Koontz has played this card a few times. Instensity has a last minute revelation that the Serial Killer is a young rising star in a local police force, Dark Rivers of the Heart has a murderous FBI agent who kills people he feels are too good for the world, etc.
Animorphs played with a variant of this; in many cases the cops were controlled by the Yeerks and so going to them would be certain enslavement since they couldn't tell who was a controller.

Video Games

Persona 4: When Adachi answers Namatame's call to the police to tip them off about the role of the midnight channel in the murders (That Adachi commited!), then uses the opportunity to make him unknowingly attempt to commit more murders.
Ace Attorney: In case 1-2, Redd White apparently has most of the police department (and the prosecutor's office) in his pocket, meaning that Phoenix has nobody to turn for support except himself.
- And in case 1-5, the villain is Police Chief Gant, who orchestrated both Neil Marshall and Bruce Goodman's murders.
In Grand Theft Auto: San Andreas, The CRASH unit, especially Officer Tenpenny, are corrupt to the core and essentially extort the player character while turning a blind eye to the gang violence they're supposed to be fighting against, preferring to just let the gangbangers take each other out rather than do any work themselves.

Real Life