Earth is uninhabitable. Five years ago, in our darkest hour, a vessel of unknown origins came to our aid, broadcasting in all languages. Humanity’s best and brightest have set off into the vastness of space on a journey to an unknown destination - hopefully a new home - aboard the Ark.
That was five years ago.
The Ark is our home and responsibility, even if we barely know how it works. If we break it, we die. If it doesn’t produce enough food to feed the people on it, we starve. If we go offship and the Ark leaves, we’re stranded.