“Twilight’s Last Gleaming” is episode five of season one of The 100.
In the previous episode, “Murphy’s Law”, Murphy was banished from the camp but allowed to live (like that’s not going to bite them in the back in the future), Finn and Clarke got together and Clarke’s mother, Abby, was arrested – and sentenced to be executed by floating.
The Ark’s systems are failing faster than expected, so Abby’s medical skills are needed and so she is given a reprieve, and a decision has been made to kill a whole bunch of people. The person sent down in the drop pod is Raven, Finn’s girlfriend – more problems, given that Clarke and Finn are an item to a degree now – and Bellamy is not happy about the idea of being able to communicate with the Ark again.
I still can’t see why there was apparently no effort to establish a presence on the Moon. Maybe this will be explained in a later episode – or maybe it hasn’t been explained because there simply isn’t a reason.