The Sound of Drums
As the first full episode to feature Missy with her true identity as the Master revealed, "Death in Heaven" features various references to previous episodes featuring the Master. Osgood refers to previous incidents on Earth featuring the Master, including his tenure as Prime Minister in "The Sound of Drums" (2007). When boarding the airbase, the Doctor believes that he is going to the "Valiant", an airborne aircraft carrier partially designed by the Master in "The Sound of Drums". Missy uses the phrase "Oh, my giddy aunt", an expression associated with the Second Doctor.