So the Grounders on CW's the 100 aren't exactly villains but they are the primary antagonists to the Sky People who are the main focus on the show. Wild and savage where the the Sky People are orderly and militaristic, they serve as the darker, animalistic and more emotionally driven heart of the show. Octavia Blake starts out as what could be the spunky little sister character in Season 1. Anyone who has followed the show from the beginning remembers her joyful "We're back bitches!!" from way back in the pilot episode. When we first see her she's all jeans, hoodies and stick straight hair. Basically what you would expect from any Freshman in high school. By the time she becomes a full fledged Grounder it's hard to even imagine that she ever looked so clean cut. The leather separates and mohawk styled braids are overtly inspired by Mad Max and the muddy black eye makeup makes her look almost demonic. They accentuate that she has firmly separated herself from the stubborn humanity of her former Sky Crew companions.
Bellamy has kept his promise to Echo and the Grounder prisoners, returning to the Harvest Chamber to free them from their cages. They will play an integral part in the battle ahead.
Clarke and Lexa are preparing the Grounder army at the doors of the mountain. Clarke goes over the plan, which includes four teams.
One at the Dam to blow the power (Raven/Wick), one at the mines to disable the Reapers and guide the prisoners to safety (Indra/Octavia), one inside the mountain (Grounder and Ark prisoners/Bellamy), and one at the doors (Clarke/Lexa).
Raven and Wick are setting detonators to blow the power source. Their plan goes awry when one of the bombs breaks, leaving them one short. They come up with a new plan, but it puts them in danger and they are caught in an explosion.
Ultimately, they do succeed in blowing the power, but the detonator at the doors fails. Sgt. Miller attempts to make it to the doors to detonate the charge, but also fails. Lincoln manages to shoot a flaming arrow that does the trick.
Lexa leaves to take out the shooters on the ridge and returns to tell the Grounders to stand down. She has made a deal with the Mountain Men for the freedom of the Grounder prisoners, but the deal does not include the Sky People.
She retreats, leaving Clarke to fend for herself.
Inside Mount Weather, Bellamy connects with Jasper, Maya and Monty. Cage has announced that the only radiation-free zone will soon be level 5. This does not bode well for Maya. They stay the course, but are horrified to find the Grounders gone from the Harvest Chamber and Maya's father dead.
Octavia refuses to retreat on Lexa's command. She tells Indra that she doesn't trust her. Indra renounces her as second and leaves her in the tunnel alone.
Clarke stares at the doors, also refusing to leave and in desperate need of a new plan.