Ncuti Gatwa makes his first appearance as the 15th Doctor in the science-fiction series “Doctor Who,” he isn’t wearing any pants. In a 60th-anniversary special released in December, the previous Doctor — played by series icon David Tennant — subverts the show’s long-standing practice of regeneration: Instead of simply transforming into the next Doctor, he literally splits in half, bringing Gatwa’s Doctor into the world alongside him.