This was true of Mary Watson (née Morstan) in Sir Arthur Conan Doyle's stories, and after the devastating Season 4 premiere, it's also true of Sherlock — although creators Steven Moffat and Mark Gatiss inarguably gave the character more depth and chutzpah than Doyle ever did before deciding to kill her off.
Indeed, Mary's demise was such an afterthought in his novels, Doyle never even gave a cause for it — a far less poetic end than Mary's decision to sacrifice herself by jumping in front of a bullet to save Sherlock's life, having previously tried to shoot him herself.
At a press screening, Moffat admitted that the decision to kill Mary off so early (she was only introduced at the beginning of Season 3, lest we forget) was a way to keep Holmes fans on their toes.
The deep dive into Mary's past was made all the more gripping thanks to Rachel Talalay's taut direction — which not only gave us some of the show's most cinematic action scenes to date, but also supplied endless rope for us to skip with in the playful first half, before it inexorably began to tighten towards the episode's painful conclusion.
As Mary points out to Sherlock in a heartbreaking video message delivered after she's dead, she has just given the detective the hardest case of his career — a mission to save John Watson when he doesn't want to be saved.