True crime meets conspiracy theory meets Agatha Christie in The Marilyn Conspiracy, written by Vicki McKellar and Guy Masterson, and directed by Masterson. Set in the few days before and the hours after Marilyn Monroe’s sudden death in 1962, the play examines her “overdose” and reaches a very different conclusion – that Marilyn was in fact murdered, and the crime covered up by seven of the friends and physicians closest to her.

In reality the play focuses not so much on the crime itself, the circumstances of which – though we don’t see it happen – are presented as indisputable fact by the writers. The real story here is the cover-up, perpetrated by British actor and – most significantly – brother-in-law to the Kennedys, Peter Lawford (Declan Bennett). As the hours following Marilyn’s death tick by, the play becomes a sort of Twelve Angry Men in reverse, with a single character gradually convincing everyone present to collude in a terrible injustice.

Interspersed with scenes of the aftermath are moments from Marilyn’s last days, and it’s here – perhaps unsurprisingly – that the play really comes to life. It must be a daunting prospect to portray such a well-known icon, but Genevieve Gaunt is excellent as a fragile but resolute Marilyn; much like her housekeeper Eunice Murray (Sally Mortemore), the audience feels both protective of and frustrated by her in almost equal measure. Her scenes are written in such a way that we see several sides to her character – through conversations with press secretary and best friend Pat Newcomb (Susie Amy) we get to enjoy Marilyn’s sharp wit and delight in Hollywood gossip; her final encounter with psychiatrist Dr Ralph Greenson (David Calvitto) reveals her intense vulnerability; and her fiery exchanges with Peter Lawford and, to a lesser degree, his wife Patricia Kennedy-Lawford (Natasha Colenso, stepping in to replace writer Vicki McKellar due to illness) reveal the rashness that – in this version of events, at least – directly contributes to her fate.

In contrast, the scenes following her death feel a little slower, as the group meticulously go over the details of the evening and settle on their agreed story. Set as it is in a single room in Marilyn’s bungalow with a small fixed cast of characters, the play has a definite Agatha Christie vibe, especially when Dr Hyman Engelberg (Maurey Richards) begins interrogating a distraught Mrs Murray about exactly what took place. But what appears at first to be a simple whodunnit evolves into a more interesting story, examining not why each character might have wanted Marilyn dead, but instead why they might want the truth of her death to remain hidden.

The living room set, designed by Sarah June Mills, features a turntable at its centre, which rotates slowly throughout the play, giving us a sense of constant movement carrying even the most unwilling characters towards an inevitable conclusion. Because the writers present their version of events as fact, there’s no space left for the audience to make up our own minds about what happened – and as a result the play leaves us in little doubt as to who killed Marilyn. The question of motive is less well defined, however; it’s implied that she knew, and was prepared to reveal, some critical information that would be damaging to her killers. But there’s very little speculation within her inner circle as to what that information might have been, which means the play lacks the usual neatness found at the end of a traditional murder mystery. Then again, perhaps that’s to be expected in a story based not on fiction but – regardless of whether or not we believe the conspiracy theory – on fact. Either way, The Marilyn Conspiracy is a tense and somewhat timely examination of how easily hearts and minds can be changed, when reputations are at stake and a persuasive speaker is allowed to take the floor.

The Marilyn Conspiracy is at Park Theatre until 27th July.