The plot is Biblical and somewhat muddled but the main things you need to know are that Saul is jealous of David and will indeed end up being replaced by him, and David and Jonathas (Saul's son) are in love, and it will all end badly.
The staging, by Andreas Homoki is a bit uneven. The set is basically a confined wooden box which can be broken up into several rooms, and with moveable walls to adjust the spaces with psychological implications for different characters which are clearer at some points than others. The movement of walls is not unreminiscent of some of Christoph Loy's recent productions (and the bits of business with chairs reminded of Marthaler) – clearly these are in elements in current European opera production, but fortunately neither are irritatingly distracting. Homoki's management of his cast is also uneven. There are some very effective images – for example at the height of Saul's torment he finds himself surrounded by images of his wife – but equally tension in staging and movement is often allowed to dissipate. But when Homoki is at his best he really nails it – the moments following Jonathas's death, and the hollowness of the crowning of David are spot on.
Musically things began to my ear also somewhat unevenly. I think this was partly a case of getting used to the sound world and partly the effect of some kind of acoustic issue. I'm not sure if it was a problem with the Festival Theatre, or the design of the set but the soloists often sounded disturbingly thin voiced. But gradually things settled down, the drama picked up and musically things got on song. The second half I found enthralling.
Of the soloists the best work came from Ana Quintas (Jonathas) and Neal Davies (Saul). Quintas's dispair was movingly conveyed in her great Act 4 aria, Davies was, as in other work I've seen him do, a powerful presence vocally and dramatically. I was less convinced by Pascal Charbonneau's David who didn't seem to improve in Act Two. At the conclusion he sounded especially thin voiced, and I sometimes doubted his tuning – but again I couldn't be certain this wasn't more to do with acoustics than the singer. Supporting roles were well taken, with a particularly fine turn from Dominique Visse as the Witch. The chorus and orchestra that make up Les Arts Florissants played and sang to a high standard under William Christie's expert direction.
I'm not sure that I'd want an excess of Charpentier in my musical life, but I'm very glad to have seen and heard this rarity. I think there may be further tour dates coming up – if they're near you, it is well worth catching.