The Doctor is so susceptible to flattery. The Qomar, perhaps the snobbiest and hard to like aliens ever, hear the Doc singing, "I Been Working on the Railroad." Since they've never heard of music, they are utterly taken with his voice. This makes him put aside his animus toward them and embrace their compliments. He is soon the new Enrico Caruso of this part of the galaxy. It turns out, however, that there is more to his gig than simple adulation. Quite funny, but an oddity to say the least.
