This another lots of firsts story. It introduces the Sontarans (and mentions their enemies, the Rutans), who went on to become a low-level menace throughout the old and new series. It also names the planet Gallifrey, which had previously not been named. But most importantly, it introduces Sarah Jane Smith as the Doctors new companion. It does all of this in a thoroughly good, intelligent way. We see Sarah Jane infiltrate UNIT. We see her not trusting the Doctor. We see the Sontarans making deals with medieval knights, and since we have knights we get castles and fights and great old timey dialogue. It's got a wonderful pseudo-historical feeling to it, as well as some very good science fiction elements, something the show sometimes lacks. Really a vastly entertaining spectacle all around, as well as being one thats important to Doctor Who history.