“The 48” is episode one of season two of The 100.
At the end of the season one finale, “We Are Grounders: Part 2”, Clarke and other members of the 100 had been captured by the Mountain Men and taken to Mount Weather. Some of the other defenders were on the run, and much of the Grounders force had been wiped out by the dropship’s engines. Octavia was with Lincoln and the Ark had performed a forced landing (which is also known as a crash).
The Mount Weather facility is occupied by another bunch of people who thought that they were the only survivors. They also cannot apparently survive outside. 48 members, including Clarke, of the remaining 100 have been taken to the facility, hence the episode’s name. Clarke does not trust those running Mount Weather, as she is not a very trusting soul any longer. Although they seem genuinely helpful, one thing stated rang false.
Kane is leading the survivors of the Ark who crashed into the planet, so he is now in charge. Which means that Bellamy isn’t.