Day 21 The 100 Series, Book 2.

It has been twenty-one days since the hundred landed on Earth. They're the only humans to set foot on the planet in centuries ... or so they thought. Facing an unknown enemy, Wells attempts to keep the group together. Clarke strikes out for Mount Weather, in search of other colonists, while Bellamy is determined to rescue his sister, no matter the cost. And back on the ship, Glass faces an unthinkable choice between the love of her life and life...