Chase (Smith) and her pal Tibb (Sudik) spend weekends picking up men in the cheesy nightclubs in their backwater town. Chase wants to leave town, but needs cash to fix her car. So Tibb makes a wager: if Chase seduces Gabe (Kirby), the prudish son of the pastor (Carter) of a strange cult-like church, she'll fix Chase's car. But thoughts that this might be just another notch on the bedpost quickly leave Chase's mind when she's taken into the church and finds out that Gabe is only 17.

There's a nicely sassy tone to the film's early scenes, in which Chase and Tibb maraud shamelessly through town, gleefully offending everyone in their path and then going out of their way to make sure there isn't anybody they haven't shocked. But once the plot kicks into gear, things begin to feel strained. This is mainly because Tibb has had a crush on Gabe since she was a little girl so, as Chase starts to fall for him, we know it'll go horribly wrong on a variety of levels.

Watching the chaos unfurl is only sporadically engaging because our sympathies are so one-sided. Fortunately, the characters are just about complex enough to carry us through, even when the actors dip into parody. The teens in the cult are earnest and pretty hilarious, especially when they indulge in some timid rebellion. And filmmaker Denmark delights in cutting away to give us a glimpse of each person's private peccadilloes.

But the real problem is that this silly moralising adds an increasingly preachy tone to the film that sits at odds with the otherwise gleefully naughty plot. This also means that the film's emotional climax feels somewhat contrived as a result. But overall this is a sweet, funny little movie that shows considerable promise for both cast and filmmakers.

Bill (Robert Hill) is a self-proclaimed "simple person" living with his pragmatic wife Maggie (Deakin) and short-fused son Karl (Robert's real-life son Robin). When Karl finds out that his girlfriend (Kerry Peacock) is pregnant, it almost takes his mind of the shady, increasingly messy business dealings he and his dad are involved in. And their friends (Schaal, Way, Kempner and Smiley) aren't helping at all. So as trouble brews with their bosses in London, murder becomes the only option.

Filmed and played in a low-key, naturalistic style, the movie draws us in through its sheer ordinariness. These are believably real people, so nicely underplayed by the cast that the scenes feel improvised. And everyone looks normal, not like movie stars, which gives the interaction and paranoia a zing of authenticity. The camera darts around the scenes like our own eyeballs, following conversations while catching little details. And a clever blend of musical styles adds tension, humour and soul.

Without giving us too much information, Wheatley creates a fascinating atmosphere that lets us see past a slightly talky script and some uneven editing. Essentially this is a series of vignettes that feel like jokes leading to punch-lines that are often witty and sometimes horrific. The film is efficiently claustrophobic, rarely leaving the terraced house in which this fractured family lives while the real story is taking place outside. All of the characters have surprises in store, and the unpredictability of these people makes them feel that much more genuine.

Another strikingly original touch is Bill's stream of outspoken opinions, as he constantly moralises and criticises everyone based on lessons learned through his own criminal activity, furious that his son seems like a waste of space. All of the actors are riveting, but the most striking, haunting performance comes from Deakin, who never hits a false note. "It's not the decisions that are tough," says Maggie. "It's the actions." And what these people do is increasingly desperate and chillingly irreversible.

It turns out that scientist Sarah Monroe (Tiffany) has been experimenting with piranhas on the Orinoco River in Venezuela, and now they're growing exponentially, from the size of a cookie to the size of a Buick in just a few days. Fortunately, they're heading for the Caribbean, and they won't be able to survive in salt water. Or so the biologists think. Assigned to help her stop the potential chaos is over-muscled American action-spy Jason Fitch (Logan), who is being manipulated by his panicky boss (Williams) back home.

Sadly, the funniest thing about this film is the cast list, because filmmaker Forsberg simply doesn't have a clue how to put them to any use at all. There's so much camp value that it's not even funny. But it's not even remotely funny. Tiffany, especially, is badly wasted as she plays every scene dead straight and doesn't say or do a single convincing thing. At least Logan is fun to watch simply for his oozing macho bravado.

And then there are the piranhas themselves, which look like they were created with paper and scissors, without the use of digital effects software at all. That said, the scenes of them surging and flying through the air are so ridiculous that you can't help but laugh, especially when they chomp on a random character on their way back into the water. Although we should thank the filmmakers that they have indulged in this nonsensical airbourne skill, because the scenes underwater are indecipherably murky.

Frankly it's more than a little annoying that this isn't more fun than this. The movie didn't have far to go before it was pushed over the edge into farce, but the filmmakers seem determined to go for thrills instead (there are none), and their inept direction and editing leave every scene hopelessly incoherent. In the end, we're actually rooting for the piranhas to take over the world. Or at least Hollywood.

Nikko (Treadaway) has been harming himself for years, and recently he's been spurred on by Stevie (Booth), a charismatic girl he met on a suicide website. His pals (Craig and Darvill) try to distract him by going out to spot rare birds in the countryside. But they don't know that Nikko has vowed to kill himself when he hits 500 sightings. And he's now on 498. The question is whether something can make him see that life's worth living.

Ironically, Stevie seems to be that very thing: she's thrilling and sexy and utterly fearless as she takes an environmentalist stand (she's a vegan but nowhere near a pacifist). Her interaction with Nikko is sweet and prickly, and we can see him beginning to see that there might be a point to life after all. Although in the film's prologue, Nikko notes that twitching is utterly pointless and makes a reference to a coming suicide.

As the story progresses, the plot gets sidetracked when Nikko encounters some extremely nasty poachers as well as a weaselly rare-egg thief, and the ominous overtones (knives and even a gun) suggest things are going to get violent before the end, which we know will include at least one death. Often this feels like forced intensity on the part of the filmmakers, who seem to throw a very wide net in their efforts to make this film Serious with a capital S.

And this grim, melodramatic tone ultimately wears us out. Fortunately, Treadaway gives a wonderfully natural performance that's engaging enough to hold our sympathy through a series of weak plot turns. Strangely, his scenes with Booth hint more at danger than romance (his friends call her Yoko). And credit should go to Booth for making Stevie such a fascinating character. But all of the pushy foreboding leaves the whole film wallowing in gloom, which makes the plot's final gyrations feel badly contrived.