“Rubicon” is episode twelve of season two of The 100.
At the end of the previous episode, “Coup de Grace”, Mount Weather’s president had been deposed by his son; the son believed that the inhabitants of the mountain should return to the surface, using whatever means necessary, whilst his father was opposed to the idea. Consequently, using the members of the 100 trapped inside as permanent radiation cures has stepped up, with fatal results for them.
Through Bellamy, Clarke and the rest discover that Mount Weather is going to launch a missile at the Grounder gathering in Ton DC. Which is presumably Washington, D.C.
They also discover that Jaha had led a group to find the City of Light, which happened in “The Survival of the Fittest”. No-one seems to have noticed that he, and the others who went with him, which includes Murphy, are missing until now. Jaha does not seem that stable.