7. The Devil's Chord
What would the world be without music? It's an important question to ask, but "The Devil's Chord" had a lot more going on than just that. When the Doctor and Ruby went to see the Beatles record their first album, they instead encountered a being called Maestro, who jealously hoards all music for themselves. After seeming to defeat both Ruby and the Doctor, Maestro was only stopped when the Beatles managed to complete a banishment chord.
There were a lot of fun moments in this episode, which helped maintain the balance of the more somber beats, including the Doctor reminiscing about living with his granddaughter. For those looking for some classic RTD-era camp, this episode is just what the doctor ordered. It's an enjoyable episode, which is more than some earlier stories can say.
However, the episode doesn't work terribly well structurally because it is trying to do too much. Maestro was primarily defeated by chance, and the tribute to the Beatles was weak, especially compared to episodes like "Vincent and the Doctor." It's a good episode, especially for building up to a major conflict with the gods, but there are better ones later in the season.