Where better to open a tour of Sue Glover's 1988 play The Straw Chair - an imaginative story of exile on the Outer Hebridean island of St Kilda in the 18th century - than at the biggest cultural venue in the Western Isles, the splendid An Lanntair arts centre in Stornoway?

Co-produced by Borderline Theatre Company and Hirtle Productions (in association with the Gaiety Theatre, Ayr), the drama explores a turbulent time in Scottish history through the experiences of four very different characters.

The play is built upon the true story of Rachel Chiesley (otherwise known as Lady Grange), an unfortunate aristocrat (played by Selina Boyack) who is kidnapped and banished to St Kilda by her unscrupulous husband in order to prevent her exposing his Jacobite sympathies. We meet her and her illiterate yet bilingual (Gaelic and English speaking) Hebridean servant Oona (Ceit Kearney) as Aneas and Isabel (a Presbyterian missionary and his teenage wife, played by Martin McBride and Pamela Reid) arrive on the island from Edinburgh.

Initially horrified by the "primitiveness" of St Kilda, young Isabel comes to be fascinated by the culture of the islanders (for whom pagan beliefs sit easily alongside Christianity) and ever-more sympathetic to the story told by the outraged, and often drunken and outrageous, Rachel. The ensuing drama - in which Aneas is pulled between acquiescence to power and an instinct for justice, and the iniquity of Rachel's plight becomes ever clearer - is truly captivating.

To a large degree, this is down to Glover's excellent capacity for dramatising her historical research. The play evokes 18th-century St Kilda and the dangerous, politically divided Scotland beyond with an impressive vividness. Liz Carruthers's fine production is neatly designed, evocatively lit and makes lovely use of recorded Hebridean music and song. It also enjoys a strong cast, although Reid is too quiet in her speech at times.

The rich role of Rachel, an indignant, righteously furious, but also erratic and unpredictable character, requires a special actor. Carruthers has one in the excellent Boyack. Her Rachel runs barefoot around the island in a tattered ball gown, clutching the modest seat of the play's title (the only chair on the island). It is an intelligent, robust, humorous and poignant performance.

Much is said of Bondagers, Glover's celebrated play about female farm labourers in the Lowlands of Scotland in the late 19th century, which was revived successfully by Edinburgh's Royal Lyceum Theatre last year. It may be heretical to say so but, on this showing, I would suggest that The Straw Chair is the better play. Neglected hitherto, this beautifully crafted four-hander deserves to be considered a modern classic of Scottish theatre.

If Glover's play is notable for its superb structure and balance, the same cannot be said of And The Beat Goes On. Stef Smith's new play for touring company Random Accomplice explores the efforts of a Scottish couple in the United States to come to terms with the disappearance of their child by way of their obsession with Sony and Cher.

Part theatre of impersonation, part bleak comedy of a tragic past, the drama might have been a cross between Peter Arnott and Cora Bissett's Janis Joplin: Full Tilt and Enda Walsh's The Walworth Farce. Sadly, however, Smith's short play lacks the great energy of the former and the tremendous dramatic depth and wit of the latter.

It's 1989, and expats Lily and Peter (played by Julie Brown and Johnny McKnight) are isolated and persecuted in a small American town, where they are suspected of being responsible for the disappearance of their seven-year-old daughter eight years ago. Broken psychologically and emotionally, they dress up as Sony and Cher and rehearse the routines and songs from the famous couple's TV show in their garage, while creepy neighbour Joan (Julie Wilson Nimmo) takes a suspiciously keen interest in their lives.

This may seem like a sketch for a play but, unfortunately, that is all that Smith has written - a sketch. The acting is fine, as is director Kenny Miller's design, but the writing disappoints in its resort to short cuts, clichés and thickly spread pathos.

Although it comes in, by necessity, at under an hour, lunchtime drama Fat Alice by Alison Carr, the latest piece from the hyper-productive stable of A Play, A Pie And A Pint, is more substantial. Moira (Meg Fraser) and Peter (Richard Conlon) are having an affair. Peter, despite his promises over 10 years, has still not found the guts to leave his wife.

As the couple make awkward small talk, there is, palpably, an elephant in the room. There is also, in broad-but-funny metaphor, an elephantine neighbour in the flat above. To say that the unseen, but often heard, Alice is fat is a bit like describing Jeremy Clarkson as merely a bit offensive. Alice fills the flat, her obesity standing in for the emotional pain of Moira, whose life and self-image have been shaped by her decade-long wait for the cretinous Peter.

A none-too-subtle, but ultimately quite affecting feminist comedy, Carr's play is directed with gusto by Joe Douglas. The production's greatest strength, however, is the performance of Meg Fraser, who is superb in her combination of sarcastic humour and genuine anguish.

The Straw Chair, seen at An Lanntair, Stornoway, touring until May 9, borderlinetheatre.co.uk; And The Beat Goes On, seen at Tron Theatre, Glasgow, touring until April 24, randomaccomplice.com; Fat Alice, seen at �ran Mór, Glasgow, transferring to Traverse, Edinburgh, April 7-11