What happens when Mabel wakes up one morning to find herself face to face with an identical clone of herself... and the copier hasn't been used at all? Utter confusion. Now, Mabel and the Mystery Shack team have to figure out how the clone got here, what she really wants, and most importantly of all, if Gravity Falls can withstand two Mabels at once...

"This is more terrifying than watching Hiccup falling into the blistering flames of the Red Death's destruction, of knowing he would lose his entire world if he was not fast enough." When Hiccup is gravely ill, it's up to Toothless to save his human before time runs out for the both of them.