Hello Norma Jeane, Park Theatre, London ★★★★

When Norma Jeane Mortensen – better known as Marilyn Monroe – died from an apparent barbiturates overdose in 1962, several conspiracy theories emerged that rejected the conventional wisdom of an accident or suicide, mostly predicated on the actress’s relationships with both President John F Kennedy and his brother Robert. Yet none of the various conspiracies ever suggested that Monroe actually faked her own death, moved to an Essex seaside town and became a foul-mouthed, loving mother and grandmother.

Yet that latter conceit forms the basis of Dylan Costello’s funny and warm comedy drama. Vicki Michelle starts off in hilarious form as the crimplene-clad Lynnie, the woman who absconds from her nursing home armed with her grandson’s credit card and flies out to Hollywood, claiming that she wants to finally reveal her true history. She is eventually joined by her grandson Joe (Jamie Hutchins) who has his own problems – life with his violent, unfaithful boyfriend had led him to retreat into a world where his best friend is an imaginary reincarnation of the “real” Marilyn.

Motivations and the truths behind them constantly shift in Costello’s frequently hilarious script. Whether Lynnie really could be Marilyn, whether she just believes she is as a symptom of dementia, or whether she is laying a trail of clues in order to give Joe an adventure that will spur him to leave his abusive relationship – each is afforded enough space to emphasise their validity. Hutchins gives Joe an endearingly impressionable air, partly dragged along by Lynnie’s crazy scheme and partly inspired to reveal her secret to the world.

In support, Farrel Hegarty as Joe’s imaginary Marilyn struggles to provide a convincing impression, relying more on the iconic dress and peroxide blonde hairstyle to convince. As Bobby, the hunky man who provides Joe with a romantic alternative to his unseen abusive boyfriend, Peter McPherson takes the classic Hollywood neurotic and lets it slide a little too often into out and out mania, making it harder to believe that Joe would be tempted to move from one psycho boyfriend to another.

But this is Michelle’s play, and it is her central performance that elevates those around her. Her Lynnie is funny, charming, and cheerily blue in her use of language, yet she is also vulnerable and more frail than she wants to admit. It is a performance that moves from verbal farce to the pensive sadness of a woman who is coming to terms with the end of her life on a sixpence. As the broad humour of the opening scenes gives way to a conclusion that is genuinely affecting, it becomes less important as to whether Lynnie really is screen idol Marilyn Monroe – it is the relationship between her and her grandson that is the most important. Michelle and Hutchins, who have great chemistry throughout, really shine as they finally talk through issues which they have been avoiding for years.

By the final curtain, Costello gives us a supposedly definitive answer to Lynnie’s identity. Whether the audience accept or reject that conclusion, the ride we are taken along provides an excellent night of theatre.