Chummy, White Bear. Photo: Headshot Toby

A man in black jeans, black scarf and black hood stalks the darker edges and corners of the stage with just the briefest glimpses of his face. This is Chummy (Calum Speed) a soon to be murderer who calls ex police detective Jackie Straker (Megan Pemberton) asking her to stop him. Straker has her own problems, not least a gin crutch, but Chummy quickly becomes her obsession and curse, threatening her own mental stability.

The play is set primarily in a dingy office from where Straker tries to run a private detective business. The rear wall is a series of blinds which lift to reveal silhouettes - Chummy creepily appears or a potential victim is seen out enjoying herself. It is a clever way of staging what is otherwise a fairly static play - its central plot device is two people talking over the phone after all. And that is part of the problem. The conversation between Chummy and Straker and their subsequent monologues need to be insightful and punchy but the dialogue at times feels odd and weighed down with simile. As a result the plot feels laboured and I'm not sure I learned much more about the mind of a murderer - or the mind of someone on the verge of a nervous breakdown.

John Foster's play has all the makings of a great psychological thriller and it has its tense moments  - ironically often when there is physical interaction between the characters - but it never quite fulfils its potential. The cast do their best with the script but I couldn't help thinking whether it might work better as a shorter radio play - the running time is 2 hours and 15 minutes with an interval.

It's at the White Bear Theatre in Kennington until 10 June and I'm giving it three stars.