1: The Unexpected (1/2)

Park Jimin received a heart a week before his death date from an anonymous donor.
However, after his surgery, he learns the name of the donor, Min Yoongi, along with a mysterious letter that promised an answer and the truth to all of this.
Attached to the letter were a key and his first clue.
With a second chance and a new life ahead of him Park Jimin is determined to seek the truth.
But who the hell is this Jeon Jungkook mentioned in the letter and what on earth does he have to do with any of this?