The Visitation: Part 2
S19, E14: A Terileptil is living in Sir John's basement and is planning to wipe out mankind with plague rats. It decides to arrange for the Doctor and Richard Mace to be beheaded.

S19, E14: A Terileptil is living in Sir John's basement and is planning to wipe out mankind with plague rats. It decides to arrange for the Doctor and Richard Mace to be beheaded.