(500) Days of Summer, Mark Webb, 95 mins, (12A)<br/>Big River Man, John Maringouin, 100 mins, (15)

'This is not a love story," a narrator advises at the start of the quirky, witty and winsome relationships movie (500) Days of Summer. But it is a love story, just a more truthful and commonplace one than those peddled in conventional romantic comedies. It dwells on a break-up instead of ending happily ever after, and has a Nick Hornby-ish level of insight into the minds of men with too many Smiths and Belle and Sebastian CDs in their collection.

Tom (Joseph Gordon-Levitt), who works for an LA greetings card company, has the romantic notion that there's a soulmate waiting for him somewhere – a belief stemming, we're told, from his early exposure to British pop music and a misreading of The Graduate. When his boss's pretty and kooky new assistant, Summer (Zooey Deschanel), mentions that she, too, likes the Smiths, he's convinced that he's found "the One". "You believe in that?" she asks. "It's love," he replies, "not Santa Claus." But Summer affects a more pragmatic approach to matters of the heart, and never falls for Tom as hard as he does for her. Even if the scenes weren't shuffled out of chronological order, so that we see the break-up before the scene in which they get together, we would know their relationship wasn't built to last.

Mark Webb, a first-time feature director whose showreel was previously full of music videos, finds a nice visual tone such that the world these two characters inhabit looks like a nice place to be – full of funky bars and parks and second-hand record shops – but not unrealistically prettified. There's a scene in which Tom walks to work, brimming with joy having slept with Summer for the first time: fountains begin spurting in the background, a cartoon bluebird alights on his shoulder, and passers-by join him in performing a dance routine that is all the more joyous for having not been too meticulously choreographed.

It's important that the film isn't too glossy because one of its themes is the disjoin between real relationships and the illusory, idealised kind of love promoted by Hollywood. There's a clever use of split-screen to make the point; the left half shows the fantasy version of how an evening goes when Tom is rehearsing it in his head, while what actually happens is played out on the right-hand side.

Deschanel, US indie cinema's current go-to actress for quirky love interest roles, does very well in what is a deliberately under-written part. (Her karaoke rendition of Nancy Sinatra's "Sugar Town" alone will be worth the admission price for many.) The whole point about Summer is that we never really get to know her, because Tom only thinks he does.

The film is smart and uncynical enough to know that Tom's problem isn't that he's a romantic, however; he just happened to fall for the wrong girl. It's left to Tom's preternaturally wise kid sister, played by Chloe Moretz, to point out that "just because she likes the same bizzaro crap as you, doesn't mean that she's your soulmate".

On the subject of the things Tom likes: lots of young men do define themselves by the culture they consume, but it can be a problem for a film to define its characters this way. It's cool that Tom wears Joy Division T-shirts. The Smiths references? A bit obvious, but okay. When he makes a big show of reading an Alain de Botton book, however, you can't help feeling a little disappointed in him.

Big River Man is about a very different kind of romantic. A gripping documentary – positively Herzogian in its wry examination of the borderland between quixotic obsession and insanity, and of the pitiless indifference of the natural world – it charts a 2007 attempt by Martin Strel to swim the length of the Amazon.

An overweight, heavy-drinking, 52-year-old former professional gambler turned flamenco guitarist, record-holding endurance swimmer and Slovenian national hero, Strel is an ebullient, larger-than-life character. At least, he is in the early, scene- setting part of this film. Towards its end, he is a broken man with a dead-eyed stare, being spoon-fed by his son.

Strel had already swum the Danube, the Mississippi and the Yangtze (on a diet of two bottles of red wine a day), and we learn something about the private demons that might be driving him to such extremes. But the Amazon expedition is promoted as an act of ecological activism designed to raise awareness of the plight of the rainforest. "He's just like Jesus," his friend and navigator says at the outset of the trip. Three thousand miles downriver, while Strel is maniacally scratching at subcutaneous larvae, this friend is fully raving in the manner of the acolyte to Kurtz whom Dennis Hopper plays in Apocalypse Now – a film with which Big River Man shares several individual images and an all-round hallucinatory quality.

Also Showing: 06/09/2009

Tricks (95 mins, (12A)

Poland's official submission to the foreign film category at the Oscars was this gentle, loveable, slightly bittersweet comedy about a boy's efforts to tempt fate into returning his estranged father.

Bustin' Down the Door (96 mins, 15)

A surprisingly dramatic documentary about the long-haired Australians in 1970s Hawaii who helped turn surfing into a professional sport.

The Red Baron (129 mins, 12A)

Charting young Manfred von Richthofen's progress from aristocratic flying ace to disillusioned Luftstreitkräfte poster-boy, this expensive-looking film seems inauthentic and is grounded by a trite script, awkwardly performed in English by a mostly German cast.

Greek Pete (75 min, 18)

Plotless but naturalistic, frank and fairly hardcore study of the day-to-day life of a London rent-boy.

Passchendaele (114 mins, 15)

Canada's biggest-budget film ever. This First World War drama is written and directed by Paul Gross of Due South fame, who also stars. Not screened to critics, but reportedly more Pearl Harbour than Saving Private Ryan.