Running away to join a circus always seemed to me a rum thing to do. As a child I found the trick ponies grotesque, the lions a bore, and the clowns with their shaving-foam pies an insult to juvenile intelligence. Thirty years on, things have changed, evidenced last season by the lavish, sophisticated Cirque du Soleil (returning soon to the Albert Hall). By contrast, this year brings a handful of go-it-alone practitioners who've whittled circus art to a skeleton of its blowsy self, recovering its lost heart and its lost innocence.

Le Cercle Invisible is a mix of circus skills, humour and illusion, delivered with a gentle-mannered humanity that carries it beyond mere spectacle. Jean-Baptiste Thierree and Victoria Chaplin have been creating serial versions of their "nouvelle cirque" since they married in 1971, gradually paring down the personnel until it's now just the two of them and a gaggle of rabbits and ducks. These latter specimens, saved from a fate in the freezer, improvise nightly in a musical turn that never once compromises their quacking dignity.

Thierree, with his long, woolly grey hair, apple cheeks and goggly eyes, cuts the figure of a classic idiot savant. He munches lighted candles and a corner of his tummy glows. He juggles deftly with an armful of oranges, but even better with specks of dust. His conjuring is sufficiently refined for him to fake failure to nice effect, his Gallic cries of "Hup! Hup!" sometimes producing the goods and sometimes not. (To my son's delight we got a joke-shop dog turd instead of the promised dog.)

More original are the purely visual gags which present Thierree in wild variations of his own hairdo, or as a walking parody of co-ordinated dressing. At one point he appears bizarrely clad in a suit of pictorial tapestry, sits on a tapestry stool and dons tapestry specs to do a spot of needlepoint - a touching vision of so large a man.

Chaplin, the sylph-like third daughter of Charlie (not granddaughter as her looks suggest), is made of sterner, stranger stuff. Floating into view in one weird costume after another, she transforms each year before your eyes into a range of elaborately animated creatures - a ghostly horse and carriage, a strutting peacock, even a giant shark which launches a mean attack on its creator, leaving her spooling yards of red satin ribbon on to the stage in a puddle of death. (Her trapeze act, by contrast, is all simplicity, a celestial celebration of swoop and swish, her hair almost brushing our noses as she flies from her heels.) Both performers repeatedly defy the conventions of illusion by revealing the mechanics as they go. And it's this, the guilelessness of a child, that is finally so beguiling.

One faint disappointment is that so much of Le Cercle is not new to a London audience. While Chaplin's eerie concerto performed on a saw could survive many incredulous hearings, Thierree's Pearl Fishers duet mimed by his kneecaps is unlikely to provoke delirium twice. Yet I'd take out a season ticket for a show that, for its short two hours, so exquisitely restores the balance of goodness in our mad, bad world.

Comedy comes in a minor key for the Russian clown Slava Polunin, whose Snowshow I caught at the end of a short tour before the London run. Though he dresses in yellow rompers and is a virtuoso of silly walks, Slava's landscape is a bleak Siberian winter of the soul, his sphere of dramatic reference more Sam Beckett than Grock. What do you expect of a clown who makes his first appearance gloomily contemplating a noose and tree, but who, on hauling in the extravagantly long rope, finds at the end of it another clown's head in a noose?

The stooge, Brazilian clown Angela de Castro, looks like a cross between a bag lady and a very depressed panda. She waddles and she looks, blankly, at him and us. We crease up. And that's it. It's Slava who is the man of action - feeding on an obsession with housework and doing battle with giant cobwebs, falling in love with his own right arm in a tenderly extended version of the coat-on-a-hat-stand gag, belching smoke through his own top hat as he impersonates the train that cruelly bears his love away. Introspection and despair are downbeat themes, but the final resolution (which literally blasts you out of your seat, another reason to keep small children at home) brings a wholeness worthy of a Tolstoy novel. Laughter and tears have never been harder to distinguish.

'Le Cercle Invisible': Mermaid Theatre, EC4 (0171 236 2211), to 12 Jan. 'Snowshow': Peacock Theatre, WC2 (0171 314 8800), 9-30 Jan.