Though Mark Kenward’s Nantucket is a “solo show,” it’s a communal experience. In fact, his voyage back to the iconic island engages audiences so thoroughly one critic noted that “the only evidence anyone was even breathing was an occasional gasp.” Kenward, who’s created seven well-received one-man performances, sails far back into his childhood to explore the contrasts between the bright summers and harsh winters of the vacation paradise where he grew up, including delving into a shocking act of violence committed by his mother. After a sensational extended run last spring that wowed both audiences and critics (“Astounding, funny, and breathtaking” raved the Contra Costa Times), now Nantucket returns to The Marsh Berkeley for two special encore performances benefiting the theater’s new Discovery Series.