The Rubenstein Kiss, Hampstead Theatre, London

On 19 June 1953, Julius and Ethel Rosenberg were executed in the electric chair for passing atomic secrets to the Soviet Union. They went to their deaths proclaiming themselves the innocent victims of McCarthyite hysteria, the scapegoats needed to whip up support for the Korean War. Though documents released in 1995 prove Julius's involvement in espionage, unresolved issues remain. Was a fair trial possible in the climate of the time? Was enough moral weight given to the fact that, when the information was passed, the Soviet Union was an ally of the US in fighting the greater foe,Nazi Germany?

The couple surface, semi-fictionalised, as Jakob and Esther Rubenstein in The Rubenstein Kiss, a long, flawed but ambitious piece by James Phillips, who also directs this well-acted premiere at Hampstead Theatre. It seizes on another troubling feature of the case. The prosecution hinged on the testimony of Ethel's brother, David Greenglass, who later admitted that he had falsely implicated her to protect his family. How would a child feel about having been spared at such a price? And what about the Rosenberg children? There would be a need to believe that the ideals for which they had been orphaned were untainted by lies or moral blindness.

The play begins in 1975, when Matthew (Martin Hutson), a law student, and Anna (Louisa Clein) meet by chance at an exhibition before the iconic photograph of the Rubensteins snatching a last embrace in a police van. The "kissing cousins" ploy (hope offered by the love that blossoms between second-generation members of a divided family) points to thepair's connection to the events we see in the scenes that cover the years 1942 to 1953. If contrived, it allows Phillips to explore, with empathy and insight, the inheritance awaiting the children of targeted activists.

The piece, though, is hit and miss. The portrayal of the Rubensteins is adroitly ambivalent. Beautifully captured by Will Keen, Jakob's idealistic fervour has to be balanced against his refusal, under interrogation, even to look at documents charting the crimes of Stalin. His passionate relationship with Samantha Bond's anxious but strong-willed Esther is at once touching and suggestive of a worrying égoïsme à deux.

Yet the treatment of the treacherous brother (Alan Cox) is too soft. It is set up early on that he has a problem with words, which means that his daughter can be too easily reconciled to him (and think twice about her suicide attempt) when he reveals that he thought the interrogators meant something else when they asked if Esther was "cognizant" of Jakob's activities. This verges on sentimentality. Marred by some clunking exposition and baffling intimations, the piece trains a sharp light on the benefits and liabilities of the legacy at its centre.

To 17 December (020-7722 9301)