After both Romana and Prince Reynart are taken prisoner in castle Gracht. The Doctor comes up with a plan to rescue them the conversation goes like this:
Doctor: Someone on the inside opens the main gate and a group of our men under cover of darkness take the guard by surprise.
Zadek: But who’s going to open the gate?
Zadek: One man alone?
Doctor: No, no, no, no. Once man and his dog, come on K-9.