A zombie lurches into a Wild West saloon. "What is she wearing?" says one semi-clad cowboy barman to another, in a camp English accent. "What's she come as?" the other drawls. Billed as the first "zombie-western", though I'm sure it isn't, Shezad Dawood's 55-minute film, entitled Feature (2008) has its moments, but they are few. Despite the presence of artists David Medalla and Jimmie Durham in cameo roles, despite the musical interludes, the gore and the wigwams parked in some home counties country park, the film is a bore.
There had already been one or two sticky moments in Altermodern, the fourth Tate Triennial. First there was the young woman who rode to her own death on the dildo see-saw at the Sugar-Tits Doom Club, in Spartacus Chetwynd's Hermitos Children (subtitle: The Case of the Poisoned Dildo). I think I've got that right. You lounge on some cushions like a pasha while various orgiastic happenings unfold on a wall of monitors. There are lots of naked women. One character describes the heated, although somewhat confusing, action: "They are having extweme thexual weleathe". The lisp is a nice counterpoint to the head-banging death-metal soundtrack. At one point a sacrificial pig is passed around at a nightclub orgy, and there are some inexplicable goings-on in a Jewish restaurant, with a sort of Busby Berkeley routine to round things off. Spartacus Chetwynd, I should point out, is a woman.
Then there was Giantbum by Nathaniel Mellors. This somewhat alarming if not actually insanitary "video installation with animatronic sculpture" entails a journey through a felt-lined room, where a bunch of people are filmed rehearsing a play about being stuck inside God's bottom. There is a lot of bad acting and declaiming, a succession of dreadful puns, gags about a time-travelling Doctor Poo and Father Shit-mass, and some mock golden showers. Imagine the 120 Days of Sodom redone as panto.
On a second screen, we get to see bits of the same play enacted in costume. Eventually one ends up in a bright white space where three identical animatronic heads groan, quiver and roll their eyes. They look like the decapitated Ian Holm in the first Alien movie. In fact, they seem to be modelled on the artist, who also plays a character called the Truth Curator in this coprophagic, blasphemous mash-up, which I suppose is in the tradition of Alfred Jarry, Antonin Artaud and the Marquis de Sade, but without the appalling relish and literary skills of any of them.
It seems absurd to bring in the writer WG Sebald at this point. Yet it is Sebald who Nicolas Bourriaud, the curator of the triennial, invokes, laying out the territory of what he calls the "Altermodern", an "other" modern – a rootless modernism for the 21st century, a synthesis of modernism and post-colonialism, in which the artist "turns cultural nomad". With their detached emotional tone, Sebald's books take us on journeys around Europe, into the past and across the uncertain terrains of memory, history and fiction.
"There are no longer roots to sustain forms, no exact cultural base to serve as a benchmark for variations, no nucleus, no boundaries for artistic language", says Bourriaud. Sebald's writings follow a similar wandering path, as do the real and imaginary journeys of the artists here. To which one might add one's own journeys around Altermodern and beyond it. It feels to me like a truism – but newness isn't the point.
The show has its longueurs, but it is also the richest and most generous Tate Triennial to date. It is also the best-installed. There are clean, elegant rooms as well as clutter, a wide range of objects and installations, dramatic turns and quiet spaces. We go from Mark Darbyshire's child-friendly, colourful and deliberately inane Culture Palace at one end of the Duveen galleries, to a gigantic atomic explosion of glittering stainless steel pots and saucepans, by Indian artist Subodh Gupta, at the other. Beyond this, the voice of a mad conspiracy theorist delivers a lecture about Jewish Masonic symbols and American banknotes from his cluttered living room, in a section taken from Mike Nelson's 2004 installation Triple Bluff Canyon.
Simon Starling takes us on a detour around a collection of archival photographs of the young Francis Bacon's design studio. Starling has had an elegant desk designed by Bacon in about 1930 rebuilt – not once but three times. Each version is based on photos taken with Starling's mobile phone, and forwarded to different designers. The desks themselves have been transported back to London to stand on their travelling crates in the gallery. We also visit a 1960s light show, where all the liquid crystal slides have congealed into slow-moving or inert forensic traces, in a work by Gustav Metzger, who once designed light shows for the band Cream. This almost works as a kind of self-portrait, but only if you know the details of Metzger's long and interesting life, and his importance as a European artistic presence.
I sit at a desk in a room filled with shadows, listening over headphones to Tris Vonna-Michell hesitating over a description of a weird trip to Detroit. There was a girl, there was a car crash. The shadows in the room are like shards of something that cannot be put back together. Among them are images: a deserted colonnade, crumpled typewritten pages, highways through a city. At another desk, one sits and listens to Olivia Plender's description of the relationship between Robin Hood and the various splits in the scouting movement in the early 20th century, and how that eventually led – via digressions on EM Forster, the Kibbo Kift and the archives at the Whitechapel Gallery – to a troubling faction called the Green Shirts (not a million miles from the fascist Blackshirts), who railed against the British Credit System in the 1930s (one of their number fired an arrow at 10 Downing Street). On the table, there are last week's newspapers, with their credit-crunch headlines. The point circuitously being made is not so different from that of the mad, anti-semitic conspiracy theorist in Mike Nelson's installation. Everything is connected, they both say. We just need the key.
As paranoia kicks in, I visit Joachim Koester's hashish club, where a film of cannabis plants judders and flashes by on the wall, like a mad scribble of jumbled thoughts. What this exhibition might really be about, I think, is the imagination, and the kinds of freedoms artists allow themselves. This is about more than transgression, or style. It is about an attitude to what it is possible for an artist to do, and about going beyond genre. There will always be stronger and weaker artists, more or less interesting art; Altermodernity won't change that, but Bourriaud makes the territory feel open and interesting again.
Which brings me to Lindsay Seers's Extramission 6 (Black Maria), one of the real finds of this exhibition. Seers shows a semi-autobiographical, quasi-documentary film about her life, screened in a mock-up shed whose design is a copy of Thomas Edison's Black Maria, his New Jersey film studio. The story is implausible, troubling, and beautifully told by different narrators.
As a child, Seers is so overwhelmed by visual stimulus that she cannot speak. As soon as she sees a photograph, she decides she wants to be a camera. She uses her mouth as the camera, and goes about with a black bag over her head. As she grows up, Seers stops being a camera, and wants instead to be a projector. She wears a model of Edison's studio on her head, projecting the movies in her mind. She struggles to illuminate the world.
The whole story is both dreamlike and moving. How much of it is true? There are interviews with Seers's mother and with a psychologist. Are they really who we think they are? As I staggered out, someone muttered "What is she on?"
