“The Girl Who Died” is a wild roller coaster ride as we soar into silly and plummet into serious. The Vikings are goofy fun while the villains stand around and don’t do much. Yet none of that matters because this is Ashildr’s story and Maisie Williams’ portrayal of the fey girl is so captivating that everything else falls away. Writer Jamie Mathieson’s script has an unfortunate tendency to jarring leaps of tone but Ashildr’s character is so finely crafted it is easy to forgive any missteps. She is a fully realized human being, both complicated and fascinating to watch as she trips over her foolish pride and frustration and radiates fierce loyalty for her tribe.