“What? …What?” were David Tennant’s first words when, to everyone’s surprise, including his, he regenerated out of Jodie Whittaker as the Doctor, and not Ncuti Gatwa, as we all expected. For three episodes the 10th Doctor is back as the 14th, before Gatwa takes over at Christmas, and in his first proper episode – after a neat appearance in a Comic Relief sketch – he has moved onto “Why?”