Even after reading a description of An Ark, this new play by Simon Stephens (The Curious Incident of the Dog in the Nighttime) remains mysterious. “From quiet sorrows to breathtaking beauty, the mystery of human experience unfolds before you,” we’re told. “Four guides shepherd you into an understanding of the human condition, encompassing the joy of first love and the inevitability of death.” One thing is certain: An Ark is the first play to blend the physical and digital worlds in a medium called “mixed reality.” Ian McKellen shares the stage with Bridgerton’s Golda Rosheuvel and the younger talents Arinzé Kene and Rosie Sheehy. Sarah Frankcom directs. —Jeanne Malle