First of all, sorry if I make any grammar mistakes, English is not my first language.
This is a re-creation of the full drama, It's like watching the drama in two hours, the purpose of this special is to put focus on the relationship between Lee Sun Joon and Kim Yoon Shik (Kim Yoon He), you're not going to see anything related to the Geum Deung Ji Sa or the Red Messenger, and because of this, several scenes were cut but still I think that the story make sense.
They didn't put so much effort in showing the friendship between Joseon F4, and you should forget the wicked Ha In Soo because he appears only once in two hours.
The description says that there are interviews and behind-the-scenes but to be honest, I didn't see them, perhaps it was because I saw it in a Korean film festival.
That's it. If you already have watched the drama, you're going to see again those beautiful moments of Sun Joon and Yoon Shik <3 and If you are not sure about the plot, or 20 episodes are too much for you, just enjoy this special ^^