Coughlan brings her own radiance to Joy, her beautiful round face and bright blue eyes shimmering with tears, her hair a halo of gold, framed by a bright red beret and matching jacket. Visually and emotionally, she is a shining symbol of the season, its warmth, yearning, and heartache. And in the end, she is the star this episode demanded.

Joy to the World was Steven Moffat's meditation on loneliness, showing how the Doctor and every character are worse for it.

Joy's ending might seem a little morbid for a Doctor Who Christmas special, but the show's got form – in fact, Joy's sacrifice is very reminiscent of Kylie Minogue's guest appearance in Voyage of the Damned, which saw her character Astrid sacrifice herself to save the Doctor, dissipating into atoms.