This episode was difficult to follow on the first viewing, and a lot of people had problems with it. Based on a re-watch and some thoughts of my own, here’s the basic idea:
The Doctor takes Clara to Akhaten, a system of seven worlds around a single star. People live in the rings that orbit a large red gas giant, rather star-like itself at times and apparently a source of confusion, but the Doctor does point at it and say ‘planet’ which should be pretty conclusive. Anyway, at the Festival of Years the people give their songs and stories to the god-planet, which feeds on them. At times the gifts are not enough, and the planet takes the youthful Queen of Years, repository of legend, as sacrifice.
This is one of those times. The god wakes, and takes Merry. The priests are unable to put the thing back to sleep. The Vigil, some kind of enforcer group, attempt to force Merry to be sacrificed. The Doctor and Clara save her, leaving the god-planet hungry. It reaches out to take what it needs, putting the people at risk. The Doctor offers himself. Somehow even he is not enough. Clara, returning, offers the thing her history – and more, the future her mother never had. An infinity of days unlived. The god-planet gorges, and presumably dies. The residents of the Rings are safe.
I am one of those people who places a great deal of sentimental value in things. I was hurt when the Doctor made Clara give up her mother’s ring. But at the same time, I’m quite certain I would do the same to save a child. It gives a new perspective on the notion of value: I wouldn’t give up such a thing for casual fun, but for life or death there is no question.
Clara will always have her mother. She doesn’t need things to remind her.