The cops have found the perpetrator of the crime they're investigating. The thing is, he's got a gun--perhaps one whose magazine is dripping with ammo. Or he's got a bomb. Or he's poured petrol (gasoline if you're American) all over the place and has a lighter.

The sequence goes like this:
#Perp yells at lead detective something along the lines of, "I'm gonna take you all down!"
#Lead detective says "Don't do anything stupid," and then "We can talk about this".
#A discussion follows, during which the detective feigns sympathy for the perp and the perp confesses to the crime.
#Eventually, the perp drops the weapon and then comes out with his hands up.
#The detective drops the sympathy, slams the perp against the wall, and handcuffs him.

A DirtyCop will probably do worse than slamming perps against the wall.
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!!Examples
* ''LawAndOrder'' and its various spin-offs use this trope liberally.
* ''{{CSI}}''
* The American version of ''Series/{{Life On Mars|2008}}'' had Sam Tyler do this at one point.
* A letter-perfect example happens to the hero of ''Conspiracy Theory''.
* ''CriminalMinds'' does this quite often, but they usually end a lot less happily, as the unsub usually ends up [[spoiler: blowing his own brains out, or ignoring the cop and goes trigger happy, leading to a similar demise.]]
** One particularly depressing episode had a war veteran who believed he was still in a war hiding from the enemy. The cops surrounded the place with snipers and told him to surrender, and the veteran saw a small child, and ran toward him telling him to get away or he'd be killed. [[spoiler: The cops thought he was going to kill the child, so they killed him before he could get to him.]]
* ''{{Comicbook/Bookhunter}}'' uses this in its BatmanColdOpen. When the library police apprehend a book thief, they find him dousing the books in gasoline, holding a lit match, and threatening to burn everything down if the cops don't back off. Agent Bay solves this the CowboyCop way: [[spoiler:he uses the recoil from a shotgun to propel himself across the room, then shoots the perp at just the right angle that his blood spatter extinguishes the match]].

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