Running Gag: the line "Ee, by gum, he's a bad 'un."
DM: (Looking at sign on door that reads C.H.M.F.F.G.) Look, Penfold...look what it says on that door.
Penfold: Oh, yes..."Chi-muff-guh".
DM: "Chi-mu--" No, Penfold, it stands for 'Car Holding Magnetic Force Field Generator'.
Penfold: 'Cor...how'd you figure that?
DM: Mm. (confidentially) I read the script.
Penfold: Ooh, naughty Danger Mouse!
The title, while playing on one of Penfold's vocal gimmicks, alludes to the SF story "Journey to the Earth's Core".