Season 4, Episode 10: Season Finale
On the eve of her stepbrother Peter's suicide, Ruth is visited by his partner Angela, an ex-MI5 officer famous for her operational accomplishments. Angela asks Ruth for help in procuring evidence that proves MI5, and more specifically Harry, assassinated Princess Diana.
Ruth dismisses the idea, but the following day Angela walks in carrying a bomb big enough to explode Thames House.
Missed this one? Trying to catch up? Read our recap of this episode here.