 It becomes crucial in the climax of this episode, used as a plot device to reveal the Master's identity,

This episode features the return of former companion Jack Harkness, who was left behind by the Doctor during "The Parting of the Ways" (2005) and ended up the main protagonist of the spin-off "Torchwood" before learning of the Doctor's presence in the series 1 finale "End of Days".Harkness uses the Doctor's severed hand from "The Christmas Invasion" as a "Doctor detector".The hand is encased in a jar that bubbles when the Doctor is near.