Nation, National Theatre, London<br/>The Line, Arcola, London<br/>The Priory, Royal Court, London

The National Theatre's adaptation of Terry Pratchett's 'Nation' makes for a poor follow-up to 'War Horse'. More entertaining is a gothic-horror take on the midlife crises of the middle classes

The National Theatre is faced with a mission impossible: trying to match its own track record. War Horse, its previous family show, featured some of the most breathtaking puppetry I've seen, and the production's West End transfer remains a phenomenal hit more than two years later.

To compete with that, selecting a top-calibre children's book as the starting point would be smart. Instead, we get Mark Ravenhill adapting Nation, Terry Pratchett's mildly schlocky saga about a shipwrecked posh Victorian girl named Daphne (Emily Taaffe) who goes semi-native with a nice tribal boy called Mau (Gary Carr). He is the lone tsunami-survivor on his native island. Joined by others from this remote archipelago, they forge a community and fight off death.

The setting, by director-designer Melly Still, has a beautiful simplicity at its core: a hillock of an isle made of dark wood, like an overturned hull, or the curving edge of the Earth. Elsewhere, though, Still is not consistently inspired, and those who remember her work in Coram Boy, for the NT in 2005, may be disappointed.

For the opening tempest, she borrows Peter Brook's vintage idea of a miniature ship. Though a fine, three-masted model, it's combined with a billowing polythene sheet that looks incongruously tacky. On top of that comes a multimedia overload: huge translucent screens showing film footage of actual waves, at odds with the less-is-more principle.

Those screens also weaken the theatrical magic of underwater scenes. The actors, on invisible aerial wires, spin in slow motion behind video-projected bubbles which, alas, make the whole thing look pre-recorded. Couldn't the National do a U-turn and have a minimal-media fad for a bit?

To give Ravenhill some credit, he pares away sluggish passages from the book and adds some amusing exchanges. Jason Thorpe is droll as the deadpan, insult-lobbing parrot: hair spiked, strutting in a corset, like a weirdly spasmodic wind-up toy. The young leads, however, are no more than amiably lively.

Nation touches on major issues: the loss of religious faith, shouldering responsibilities, working towards mutual understanding. Yet the anthropologically bogus society created on stage is sometimes cringe-making. The fusion of half-baked tribal dancing with sentimental European ditties, such as "Happy Birthday", is especially cheesy.

Then there's Daphne's discovery of the sacred cave city of Mau's ancestors: a civilisation which, we're told, invented telescopes in the Ice Age but managed only to carve silly, grinning cartoon gods on its temple walls. At this point, I developed a severe pout.

The artwork, surely, ought to be better in Timberlake Wertenbaker's new biodrama The Line. Set in 19th-century Paris, it depicts Edgar Degas and his talented, working-class protégée Suzanne Valadon, who started out as an acrobat and promiscuous artist's model. Wertenbaker traces this odd couple's turbulent friendship over several decades. The strong-willed young woman craves Degas' approval, then rejects his old-school teaching methods. She ditches the hard graft he advocates in favour of a wealthy husband, yet eventually returns to boho Montmartre.

Characteristically, the Arcola has garnered an impressive company. Henry Goodman plays the dogged bachelor with comical pedantry, a fiery temper and a flicker of romantic regret. Selina Cadell provides support as Degas' starchy housekeeper. This premiere is staged in the round, under garret skylights, by Matthew Lloyd (who recently directed Goodman in the pitch-perfect hit Duet for One). Yet while the maître praises the fluid lines of Valadon's sketches, Wertenbaker's writing feels stilted, and the storytelling is jerky, insistently spelling out its themes like an academic lecture. Degas' ditching of his Jewish friends, during the Dreyfus Affair, is illuminating, but obtrusively slotted into the plot. Goodman strains to invigorate all this, and Smart, with little stage experience as yet, needs lessons in vocal projection. She half-shouts everything, leaving one thinking Degas must, in fact, be going deaf rather than slowly losing his sight and, suddenly, his marbles.

In The Priory, Michael Wynne's new dark comedy, a hooded figure keeps flitting past the window of the gothic pile that Jessica Hynes's Kate has rented for New Year. Thirtysomething and recently jilted, Kate is hoping to have a mellow weekend in the country with a few old friends. This, of course, is doomed.

Her old flame, Rupert Penry-Jones's cute but feckless Carl has managed neither to ditch his stash of cocaine nor his TV-producer wife, Rachael Stirling's monstrously swanky Rebecca. Almost as bad, Alastair Mackenzie's Ben, a slick travel journalist who can't admit he's a lost soul, rolls up with a pea-brained beautician. Charlotte Riley's Laura is all stilettos and faux pas.

Knowingly cadging from the gothic-horror genre, The Priory is hardly entering untrodden territory, and disastrous dinner parties are old hat. (The one in Cock, in the Theatre Upstairs, gets a more novel spin.) Like Alan Ayckbourn's, Wynne's comic stereotypes – today's middle classes in midlife crises – would be perfectly at home in an upmarket TV sitcom. That said, this is an amusing evening, performed with verve and polish by Hynes, Joseph Millson as her gay friend, and the rest of Jeremy Herrin's fine cast.

'Nation' (020-7452 3000) to 5 Jan; 'The Line' (020-7503 1646) to 12 Dec; 'The Priory' (020-7565 5000) to 9 Jan