Thanks for asking. Actually there's no first part, or at least not in a way you mean it, but I'll try to be as informative and helpful as possible. This story is a fan fiction based loosely on an anime called
Suisei no Gargantia - a dystopian world, where mots of the planet is covered by water and the remnants of human race are forced to live on a giant flotillas of ships. If you're interested in Reema & Muzzle's previous adventures, you should exclusively refer to two special episodes subtitled
Meguru Kouro, Haruka, which were released one year after the TV series, in 2014.