First, a few words of advice about Klütterkammer, German artist John Bock's first London exhibition, which currently fills the Institute of Contemporary Arts. Limber up with a few press-ups, don't wear your best clothes, and leave the high heels at home. If you've no head for heights a fear of confined spaces, an aversion to making a fool of yourself, or if the sight of naked Viennese Actionists doing peculiar and insanitary things with rubber tubing or a dead pike is upsetting, get someone else to go instead and make them tell you about it later.
The trouble with this exhibition is that once you've found your way into the installation in the main downstairs gallery, there's a chance you'll never escape. You must enter on your hands and knees through a hole in a plywood wall, or up a precarious ladder through another hole above your head. The entire space is a confusing shanty of wooden tunnels, silver-foil chambers, bridges, and crawl spaces on different levels. Suddenly, in what appears to be some straw-filled farmer's den, you come across projected footage of a colony of seals basking on a rocky shore, to the accompaniment of Wagner's Parsifal.
To get here we must endure various lengthy crawls, climbs and other undignified ambulations. Doughty exploits, and arduous journeys are one of the recurrent leitmotifs here. Somewhere up a tunnel, which it's difficult not to get wedged into, is a DVD about mountaineering. Another small section of the show is a homage to the deeply eccentric French literary genius Raymond Roussel, which includes a copy of his book New Impressions of Africa, about a journey he took to that continent - or rather didn't take, preferring to be chauffeured around Europe in his luxurious motor-caravan with the blinds drawn. Some early films by Bas Jan Ader, who was later lost at sea attempting a solo Atlantic crossing in a small yacht, are also shown. We see him falling from a tree into a shallow river, and riding his bike into a canal accidentally on purpose.
Allow yourself lots of time for all this. You don't want to miss Vincent Price committing the unspeakable murders of a roll call of great British character actors in Douglas Hickox's 1973 British horror movie Theatre of Blood, screening constantly on some stitched-together patterned blankets in one of the upper galleries, which Bock has turned into a cinema. You'll have to sit on a clingfilm-wrapped bale of hay to see this grand guignol farrago. The film did upset me, not least because the characters Price dispenses with are all repellently vain, poncey critics who have trashed his career as a Shakespearean actor. Is this a warning? If I say something rude about Bock, shall I be decapitated, like Arthur Lowe; hacked to death by tramps, like Michael Hordern; or made to eat my own pet poodles, like Robert Morley? Luckily, I do not own a dog.
Or is the camped-up hamminess of the film somehow a clue to Bock's own sensibility? He is, after all, as much the master of ceremonies in Klütterkammer as he is the artist. In his multi-part installation, Bock has brought together works by other artists, and much else besides, with which he feels affinities. It is easy to see why he has included alarming works by the Viennese Actionists, a hideous man flopping over a toilet by Paul McCarthy, a video of Vito Acconci eating straw, a carved, crucified frog by Martin Kippenberger, and works by Georg Herold, Sarah Lucas and Sigmar Polke.
In the second upper gallery, there is a makeshift dormitory of screened cubicles, all hung with brightly coloured, patterned blankets. In each cell, Bock brings together unlikely artworks: a Beuys sledge and photos of satyrs by Matthew Barney; Mike Kelley and Georg Baselitz. These forced conversations and their eye-popping surroundings all work perfectly. I've seen this sort of thing done before, but rarely with such wit and perverse respect for the artworks chosen. It is all oddly harmonious and poetic, a foil to the darkened clutter downstairs.
Other references are more obscure: Rasputin's fingernails are presented in a vitrine in a chamber built like a tree house (you have to climb a ladder to get to it), and it is pretty difficult to concentrate on the vitrine of objects related to polar exploration, which are on loan from the Royal Geographic Society, because there's a DVD of The Cure's Greatest Hits blaring over it. "This is my history," Bock is quoted as saying about the show.
If this show presents his history, then it is a weird one. A farmer's son from northern Germany, Bock studied a curious amalgam of art and economics in Hamburg, and is as well known for his incomprehensible, gobbledegook spoof lectures as for his filmed performances and the objects he makes. Bock has been compared to Harpo Marx and Kurt Schwitters. Schwitters tried to build impossible things, Harpo always had the world collapsing about him - Bock is somewhere in the wreckage.
To watch Bock's own short movies, shown down in the ICA's concourse, entails further indignities: a climb to the top of yet another ladder, jamming your head into something that resembles a dog kennel, or sitting in an empty bathtub. It may sound a little strange, but at the time, all this makes perfect sense. The history of modern art shows us that the extreme, whether in terms of the work itself, the ideas behind it, or the manner of its presentation, has become a kind of normality as a creative mode. As ever, one must suspend disbelief: you know, most of the time, that nothing really bad is going to happen.
Actually, quite a few painful things happen in Bock's films. For a start, a man's head explodes but the gore is only shredded red cabbage. Bock gets mashed in a boxing match, and has the stuffing, or rather cauliflowers and cabbages, knocked out of him, along with spurts of green blood. I suppose you could call this vegetarian violence. Bock also takes a hammering as he climbs lots of scary booby-trapped staircases in a creaky house where even the frankfurters are animated, toothpaste bubbles up between the sofa cushions and chilli peppers disappear furtively, under the living-room rug. Bock falls down an endless enclosed chute, eventually diving headfirst through the sunroof of his car, which he then proceeds to drive. There are creepy experiments with yet more toothpaste, fag ends and nail clippings, and a wild ride in a wayward farm tractor. He truly suffers for his art.
Up to a point, so must we. By now my jumper is covered in straw, my clothes are wretchedly dusty and awry, and those high heels I mentioned earlier are looking definitively mangled. I feel like an exhibit myself. In fact, it is hard not to. One of the pleasures of the show is to watch other people negotiate the works. You can't tell them apart from the sculptures, and you get to meet interesting strangers crawling about in the tunnels. It all leads to a sense of camaraderie, surprise, and good humour. And don't worry about the pigeons perched on the joists overhead. They're stuffed, and are, in fact, a work called Tourists, by Maurizio Cattelan.
I was in there for hours. In many ways, Bock reminds me of a lot of other artists - and not always the ones he has included in this exhibition: one thinks of Irwin Wurm's improvised and interactive "one-minute sculptures", the earlier, rougher makeshift installational work of Jason Rhoades, Fischli and Weiss, Pierrick Sorin's little movies. But that's because art is a conversation as much as a matter of derivations, forebears, influences or strained-for originality. As an artist, you've either got it or you haven't, and the rest is bullshit.
Klütterkammer, an exhibition by John Bock, is at the Institute of Contemporary Arts, London SW1 (020-7930 3647), until November 7. www.ica.org.uk