"I like working here," says Lucy the carer for an old gentleman with dementia, "I never know who I am going to be." The elderly man is celebrated cinematographer Jack Cardiff. As the play opens, Cardiff's son Mason has engaged Lucy as a carer for his father in order to help him finish his memoir. The garage at Cardiff's home has been turned into a museum of film memoriabilia to help him remember. But all it does is confuse him. He does remember his life but he thinks it is happening now. What he hasn't forgotten is the importance and the magic of light - reflected by a prism. This captivating play, written and directed by Terry Johnson, has Cardiff at the end of his life, confusing past and present. We see glimpses of his time filming The African Queen in the jungle in 1951. He confuses his last wife Nicola with Katharine Hepburn. He mistakes Lucy the carer for Marilyn Monroe and his son Mason becomes both Humphrey Bogart and Arthur Miller. This gives the cast of four every chance to show tremendous range and they pick it up joyfully and run with it. Robert Lindsay as Jack Cardiff - still lucid in his confusion, still insightful, still able to paint portraits with words - is a delight, deft, nuanced, funny, with immaculate comic timing and verve. This is a clever piece of theatre, Lindsay shows the twists and turns of Cardiff's mind, ever the gentleman but always aware of beauty and intelligence wherever he sees it. This man will always, always be charming. This is a towering performance. Tara Fitzgerald as Cardiff's wife Nicola - and Katharine Hepburn - and Oliver Hembrough as Mason, Arthur Miller and Humphrey Bogart create the magic. We are taken so convincingly into the foul tropics we could feel flies tormenting the African Queen's cast. This is a funny play well laced with humour and a lot of it is carried by Victoria Blunt as both Lucy (a full-trained carer who completed a a course lasting three days - "Well two and a half") and a silky, shimmering Marilyn. Jack Cardiff did write a memoir called The Magic Hour. This engaging piece of theatre, beautifully written and immaculately performed, leaves us wanting to know more. Prism is at Cambridge Arts Theatre until Saturday, November 23.