Piranha

Everything About Fiction You Never Wanted to Know.
Totally original poster, we swear!

Piranha is a 1978 B comedy-horror film, directed by Joe Dante and written by director-to-be John Sayles, about a swarm of genetically-engineered killer piranhas.

Two teens break into an abandoned facility to engage in a spot of skinnydipping in the reservior, and are promptly pulled under and torn to shreds. The next day, insurance investigator Maggie McKeown (Heather Menzies) is dispatched to find them. Teaming up with local hillbilly Paul Grogan (Bradford Dillman), the two enter the facility themselves and inadvertedly drain the critters engineered by resident mad scientist Dr. Robert Hoak (Kevin McCarthy) as part of the now-defunct operation Razorteeth (in which they would be unleashed into the riverways of North Vietnam) into the river; wherein they make themselves en route to a nearby summer camp where Grogan's daughter is staying, and ultimately the ocean...

In many respects a parody of Jaws, Universal Studios initially threatened to sue until Steven Spielberg saw it himself and loved it. In 1981 it got itself a sequel titled Piranha II: The Spawning in which the piranhas fly and which happens to be the directorial debut of none other than...... *drumroll* ...... JAMES CAMERON!!!!!!

Well, sort of. The sequel endured massive behind-the-scenes turmoil and repeated re-edits, and how much of the above individual's work actually made it into the final project is open to debate (rumor has it that said individual at one attempted to break into the studio either to salvage or destroy the film). Whoever is ultimately responsible, the sequel is jaw-droppingly, gut-bustingly, narm-tastically bad; Cameron at least has enough of a sense of humor to have said for the record that it's the finest "flying piranha" movie ever made.

Midway through the filming, the director (or at least the credited one) had a nightmare; the result of that nightmare was The Terminator. So at least something good came out of this.

The first film was rather pointlessly remade as a TV movie in 1995, most notable for starring a young Mila Kunis and for reusing footage from the original film.

A more notable 3D remake called Piranha 3D eventually arrived in summer 2010, directed by Alexandre Aja. Shifting the action to Lake Victoria and ditching the Government Conspiracy set-up in favour of an equally implausible plot involving prehistoric versions of the eponymous fish, it was Bloodier and Gorier, Hotter and Sexier and generally revelled in its own silliness. A sequel to the remake, Piranha 3DD, was released in 2012.

Tropes used in Piranha include:

Reporter: "Terror, horror, death. Film at eleven."

Maggie: Well come on, let's go.
Paul: Go where?
Maggie: You're taking me up there.
Paul: Oh, no, I'm not.
[Cut to him accompanying her to the dam]