Written by and starring Shelley Mitchell, Talking with Angels tells the extraordinary life story of Gitta Mallasz. A medal-winning swimmer in the 1931 Olympics, Mallasz saved over 100 Hungarian Jews from Nazi persecution during World War II and spent her later years recording the curious series of spiritual encounters she experienced with three friends in 1943. It's a unique biography, but despite the fascinating source material and a commanding central performance, Mitchell's adaptation fails to capture the magic of Mallasz's life.

After introducing herself to the audience, Mallasz begins by sharing the details of her first supernatural experience. During an intimate personal conversation in 1943, she is suddenly addressed by a mysterious and unfamiliar voice booming from the mouth of her friend Hanna: 'It is not I who will speak with you now,' it announces. Every Friday for 17 months, this voice and three others, believed by the witnesses to belong to angels, return to Hanna's body, proffering existential advice to Mallasz and her friends.

These angelic visitations form the backbone of Talking with Angels, but although they were undoubtedly meaningful for those involved, the play fails to translate them into an interesting theatrical experience. The angels' spiritual counsel begins laughably mundane ('Be independent!' is the first heavenly injunction) and quickly becomes utterly inscrutable. And despite Mallasz's own admission of their incomprehensibility, the show is bloated with dozens of these interventions, which seem to offer little narrative purpose.

Nonetheless, Shelley Mitchell gives a formidable performance as Mallasz. From the moment she totters onto the stage, she exudes a benign charisma, and the show is at is best when she recounts the fascinating details of her admirable efforts during Nazi occupation. And despite their unintelligible dispatches, she is captivating when channelling the four angels, rendering each one with its own distinctively uncanny characteristics.

But Mitchell's best efforts do little to redeem the ill-constructed adaptation. Hampered by its focus on the voices' opaque ramblings, Talking with Angels struggles to maintain any narrative force and feels every bit of its 75-minute length.

Summerhall, 0131 560 1581, until 30 Aug, 1.05pm, £8--£10