When your Grandfather was old and dying, he had time to say goodbye to all the people he loved, and to give away the things that would normally included in a Will. He gave you an envelope. He told you not to open it yet, but to wait until you were finding modern city life was crushing your Bright Spirit.
Many years later, as a grown-up, you have a desk job at Joja Corporation. And it's crushing your spirit. So you open the envelope, to find a deed to Grandpa's old farm, in a small town called Stardew Valley.
Stardew Valley has (mostly) friendly residents but the economy's in bad shape. Perhaps as you develop your overgrown farm, you can make a real difference to others as well!
Grandpa's spirit will visit you on Day 1 of Year 3. It may be that by then you have already outmatched his own achievements on the farm! If not - well, there's no time limit. His spirit can be invited to return if you offer a diamond at his shrine.