In Strach's own words:
You start by jumping off a cliff, and if you know what to do, soon you embark on an underwater adventure! Watch your oxygen! If you don't know what to do, there are hints and even a step-by-step walkthrough which honestly takes away all the fun, so use it only if you really have no idea what to do. There are two achievements to earn, and for completing each you gain access to a special prize! (of course you can gain it immediately by checking in the .gam file but you are too noble and honest to do that, aren't you).