High in the Swiss Alps, a reclusive writer is hiding away in her study, surrounded by her collection of guns, swords and knives, in a fug of cigarette smoke.

This is Patricia Highsmith, American author of a string of successful crime novels, including The Talented Mr Ripley.

Self exiled from the literary community, ageing and unwell, bigoted and angry, she has little to live for.

Then a chirpy young man turns up - sent by her New York publishing house to try and persuade her to write one final instalment in her best-selling series about trickster Tom Ripley. At first she is reluctant to take to the typewriter again, but as the night goes on, the unlikely companions finally strike a deal.

But nothing is at it seems. The boundary between fiction and reality starts to blur, with potentially deadly consequences.

Switzerland is the name of this brilliant two-hander, written by Joanne Murray-Smith and directed by Lucy Bailey.

It stars Phyllis Logan as Patricia Highsmith. Better known as housekeeper Mrs Hughes in Downton Abbey and Lady Jane Felsham in Lovejoy, here she transforms into the scruffy, irascible misanthrope Highsmith had become.

She meets her match in Calum Finlay, who plays the visitor, Edward Ridgeway. The transformation of his character from bumptious, fair isle sweater-wearing ingenu to – well, what he becomes – is mesmerising.

Filled with razor-sharp dialogue and dark humour, this chilling play grabs your attention and unfolds into a gripping psychological thriller - much like Highsmith's novels.

Switzerland won the Best New Australian Work at the Sydney Theatre Awards in 2014.

One of the greatest writers of the 20th century, Highsmith's work includes Strangers on a Train, made into a film by Alfred Hitchcock, the Price of Salt, recently released as the film Carol, as well as the Ripley series, which have been made into several films.

Switzerland does not you go for a moment, and is well worth the journey to Bath. Don’t miss it.

The play runs at the Ustinov till September 1, at 7.45pm, with matinees at 2.30pm on Wednesdays and Saturdays. Tickets are £27.50 with concessions available. A £1.50 booking fee applies. To book, visit theatreroyal.org.uk.