Philippe V was a Bourbon prince who secured the throne of Spain using his family connections. Claire van Kampen is a writer who relied on the same method to secure a West End opening for her play about Philippe. It stars Mark van Kampen (aka Mark Rylance) as the charmingly dotty Frenchman. Philippe was a manic depressive who regarded his Spanish subjects as a puzzling inconvenience. He had no interest in governing them and preferred to laze around the countryside, looking at stars, listening to music and indulging his eccentricities. We first meet him in bed trying to hook a fish supper from a goldfish bowl. Courtiers secretly plot to oust him while the queen scours Europe for a singer capable of cheering him up. She hires Farinelli (‘little baker’), who warbles to him day and night in his rural retreat. Farinelli was blessed with the finest vocal kit in Europe but he resented working in a rustic backwater so he filled the longueurs by cultivating a chaste romance with the queen. That’s about it, plot-wise. And there’s little character development in this threadbare frock-fest.

Yet it’s a major work of art for two reasons. First, the set is among the costliest and loveliest things I’ve ever seen in a theatre. The rear of the stage has been rebuilt as a miniature auditorium with three pillared galleries overlooking the playing area. The facings are painted a gorgeous deep blue-grey with austere gilded decorations. The panelled roof is scattered with winged stars like butterflies or twinkling comets. The sumptuous restraint of this design is stunning. The lights are artfully satisfying as well. Two huge candelabra shimmer with naked flames (a nod to Wolf Hall) and they’re supplemented by conventional spots concealed so effectively I couldn’t locate them. The effect of this virtuoso display is to immerse us totally in the 18th century.

Secondly, there’s Mark Rylance. His turn as the deranged, bumbling king has no obvious antecedent. He isn’t a physical clown. Glances, tricks and comic gestures are alien to him. Atmosphere and rhythm are his materials. His pace is slow, estuarial. He has some of Stan Laurel’s ruminative sweetness but he also has the endearing, fatalistic candour of a tramp. He gives the king sudden bursts of emotion, of laughter, of sadness and anger. Because he deadpans every line it’s hard to convey the hilarity of his complaint, for example, that in Spain the Spaniards serve only Spanish food. ‘It’s all they do.’ In a fit of lust he tenderly gropes his wife and then discards her. ‘Stop that,’ he scolds her. Oddly enough, he doesn’t trigger gales of mirth throughout the show but he creates the sense that a huge laugh may come at any moment. It’s transfixing. I almost felt I wasn’t watching a ‘performance’ at all, a calculated imposture that tricks you into feeling amused, but a character study that belongs to nature, not to art. It’s like observing a child absorbed in a solitary game. He resides in this world while participating in some inscrutable other dimension which the viewer can access only through the child’s blurred reflections and responses.

Rylance, luckily, is on stage nearly all the time but whenever he departs the tension dissolves and the script settles into its meagre components and becomes a tepid love triangle featuring a singer who grudgingly accepts a king’s patronage while fumblingly assaulting a queen’s virtue. The dialogue, which makes no concession to antiquity, is funny for all the wrong reasons. An official offers Philippe a quill and parchment and asks him to ‘sign off’ on the ‘defence budget’. Oodles of cash have been hurled at this show and its 18-strong company of actors and musicians. Evita would have cost less. I hope they make their money back.

Medea, adapted by Rachel Cusk, gives us Medea as Rachel Cusk. She’s a yummy-mummy writer whose trendily bearded husband has been caught cheating and she’s jolly fed up with him and he’s jolly fed up back. Transplanting this ancient tale to the present day does it irreparable damage. Jason was a prince with a kingdom to bequeath and Medea’s slaughter of their children destroyed his dynastic ambitions. Those concepts are meaningless today so what we’re left with is a fruity portrait of a fracturing marriage with some atmospheric lighting thrown in. Medea’s spiky, pugnacious language is a treat to listen to and Kate Fleetwood is brilliantly cast as the sexy vixen whose face breathes death. (She’d make a great 007.) But the play dodges the part that makes Medea Medea. The killings are presented in an oblique and confusing way perhaps because the audience is aware that posh Islington mums simply don’t murder their kids. That apart, this isn’t a bad piece of yuppie soap. But it’s hardly Medea.