Bloodshot, St James Theatre

There is no point during Bloodshot where you can be entirely sure just what you are watching. At times it seems like a straightforward one-man show, with sole cast member Simon Slater charging around wildly in his efforts to bring the multiple characters to life. At others, it's a cabaret, as Slater whips out a saxophone and coaxes forth a few achingly good riffs.

Then, there's an impromptu magic show, complete with razor blades to be swallowed and cigarettes that appear behind audience members' ears. Finally, encompassing all these seemingly disparate elements is a gripping whodunnit twined around a very unusual love story. If nothing else, this show certainly provides value for money.

Throughout, Simon Slater is Derek Eveleigh, a photographer who has been scarred by his work for the police capturing the harrowing images of Blitz victims during World War Two. By the time the action of the play begins in 1957, he has been discharged from the force because of his drinking problem, and after some success as a freelance photographer now finds himself down on his luck, facing a rent increase he can't possibly afford. Slater is extremely convincing as this slightly sweaty, desperate man, who always has his top button undone and a crumpled mackintosh to sling round his shoulders while he wonders where his next drink is coming from.

There is a kind of escapism here akin to a good detective novel

Eveleigh's bacon is saved, however, by a manila envelope that drops through the door to one side of the stage. In it is a typewritten commission to follow a black woman by the name of Cassandra around Notting Hill, covertly photographing her. It is handsomely paid work, of a kind Eveleigh can't afford to decline. He is thus drawn into an ever-twisting plot that requires Slater to portray an Irish comic, an American jazz musician, a Russian conjuror (below right), and a London copper, as well as the ever-so-slightly camp protagonist.

The one-man format is largely successful, even if Slater's Irish accent is a little more dubious than is perhaps desirable. The section where he "chases" Cassandra around a park is likewise a little awkward, as a crowded set leaves him dashing round and round a kitchen table. A large screen behind him is an enormous help in creating atmosphere, displaying the photographs he takes while following the object of his desires and thus allowing the audience to see quite how beautiful and mysterious she is. This is also in many ways a London show - wry asides about the relative merits of Archway and Pimlico go down particularly well with a knowing audience in the capital.

The whodunnit element provides a kind of escapism akin to a good detective novel - disbelief is willingly suspended in exchange for an intriguing solution to a murder. There is a sense, however, that the pacy plot comes at the expense of the writing, with Slater's delivery occasionally hinting that the dialogue will reach heights it never quite attains.

• Bloodshot is at the St James Studio until 25 January