Remember the noughties, when Kristen Schaal was a fringe comedy fixture, scoring an award nomination in 2008 for her two-hander with Kurt Braunohler, Double Down Hearts? Those were the days of Flight of the Conchords' sitcom, when Schaal was about as wilfully dotty and insubstantial as comedy could get. Two decades on, and she hasn't changed a jot - much to our collective delight and bewilderment.

The Legend of Crystal Shell, her latest two-hander with John Roberts, casts Schaal as a half-woman, half-horse who must overcome a lifetime of shame to perform Tchaikovsky's Swan Lake for tonight's audience. The whole thing plays like it was written by a primary schoolchild high on pick'n'mix - endearing and nonplussing in equal measure. You know, the usual.

The show kicks off with Schaal confined to a cardboard refrigerator-cum-horsebox, shyly announcing herself. She emerges to recount her origin story: the progeny of her mum's unlikely tryst with a stallion named Shimmer. We meet her friend Arnold the (suspiciously dormant) mouse. She reads non sequitur poems and tries to catch fairies in a net. Because why not?

A flightier confection would be hard to imagine, and my patience was fraying until - at the blast of a trumpet - Crystal's long-lost dad makes an appearance. That multiplies the cast, at least, if not the significance, as this wannabe unicorn starts singing about the malfunctioning healing powers of his "teeny tiny horn."

So what does all this horsing about amount to? Both performers are compelling: Schaal as the otherworldly womanchild, Roberts as her Fonz-alike father. There's something almost audacious in the randomness of the plot development, as if they'd set themselves the challenge of accepting every first idea, however arbitrarily it advances the story. There are hints, if tenuous, that Crystal's late blooming might be a metaphor for menopause. If so, it sheds little light but plenty of sparkle as Schaal's alter ego grows her own twinkling uni-horn and puts a life of meekness behind her. There's nothing meek about Schaal's comedy, whose take-it-as-you-find-it idiosyncrasy is easy to admire and, usually, to indulge.