The thriller has long been in decline as a stage drama genre. This is odd, as it doesn’t in fact rely on our seeing crucial events at all, so the screen’s advantage in graphic portrayal doesn’t count for anything here. Moreover, the form can be used as a Trojan horse for more profound matters, as it is by Morgan Lloyd Malcolm in this transfer from the Hampstead Theatre’s downstairs try-out space.

Middle-class Heather arranges a meeting with down-at-heel, pregnant Carla. It is almost immediately apparent that they have a history: they were school friends until Carla suddenly switched to bullying Heather relentlessly. It is those impulses that Heather now apparently wants to hire… To say any more would entail major spoilerage, since the 75-minute piece depends on two major plot twists, the first fairly predictable, the second perhaps much less so. Put it this way: you can tell from the three dozen-odd butterflies, moths and beetles wall-mounted in glass cases on David Woodhead’s set that this household is no stranger to the killing jar.

MyAnna Buring and Laura Donnelly are now both principally screen actors. Far from hampering them, this fact strengthens their performances in this 100-seat space: they can pretty much perform as subtly as for the camera and yet feel a live audience responding to those little touches. Buring in particular has a wonderfully articulate square centimetre just between her eyebrows; she doesn’t need to go into full furrowed-brow mode, but can let the merest pucker speak for her. Director Tom Attenborough, too, has the exact measure of both the performance space and his actors’ abilities. Even my inner hair-splitter was unable to fault either Swedish-born Buring’s Cockney accent nor my Northern Irish compatriot Donnelly’s Received Pronunciation.

As the pair’s current relationship and back-story unfold, we find ourselves watching not just the suspenseful events in real time but also Lloyd Malcolm’s meditation on what shapes a person: how we may be conditioned or imprinted by incidents which make no impression at all on others, and how such dormant traits may be horrifically re-awakened. To paraphrase the play’s payoff line, these characters and performances make for a pair of very good bad girls.

Written for the Financial Times.