The Doctor:  I 'm sorry to tell you that Romana can look after herself . We 're Time Lords . Not like those innocents on Atrios , you know . Time Lords , sent by the Guardian to recover the Key to Time .
The Shadow: I know who you are , Doctor , I have always known . I have been waiting for you . I too serve a Guardian , a Guardian equal and opposite in power to the _ one _ you serve . The Black Guardian - he who walks in darkness . And you are in the valley of the Shadow !