The game starts with a ship crashing into an alien planet that seems to be fully covered by water, and you'll have to figure your way out of there.
It combines a naturalistic look at the undersea ecosystem and a scientific/technological approach to it as you have to know what it's made of and what you can take from it to go further into your investigation and your escape. It masterly makes its pieces work together offering one of the best adventures I've ever played, the need to understand your surroundings to survive leads to curiosity for its nature which will only grow further as you get into the events that led you to your current situation.
In addition to that, the depths and their lack of natural light create a terrifying atmosphere that freezes you sometimes, making you think twice if you want to go down there. And once again that works perfectly well with its other systems, crafting different tools and vehicles will give you the ability (and the guts) to reach lower levels of the sea, discovering new flora, fauna, and biomes (also new perils).
As the game goes on you start to unravel the mysteries of a rare disease that you can see affecting not only you but the fauna surrounding you as well, at the end you can escape the planet you're stranded in, or care for the disease, understand the lifeforms by their own and not only for the profit you can take of them, and help to make a remedy to help the whole planet and make the cycle of life keep going.