a game of chess between man and machine. anyone who says that simple one-dimensional chess can expose the limitations of a machine mind hasn't come across the chess-skills of a robot dog.
K-9 helps out in this adventure, especially against the Sontarons. somewhere at the end, the robot dog stays behind to look after Leila. but the Doctor reveals another K-9 which is in fact a Mark-II version, meaning that the K-9 we met, and who stayed on Gallifrey was the Mark I version.
the 4th Doctor (Tom Baker) admits that the Robot Dog is in fact his second best friends. I'm just gonna' take a simple wild guess, and say that Leila, the woman warrior is his first.
this episode marks not only the appearance of the Mark II version of K-9, but it also marks the appearance of a new companion by name of Ramona. it also marks the beginning of a long mission; a hunt for some special keys. and it takes place..., wait for it..., in the snow.