There's a traitor at Monstacademy! When Trixie returns to Monstacademy after being suspended, her best friend Gloria doesn't seem herself. To make matters worse, she's suddenly best friends with an exciting new monster. Is the new girl all that she seems, or are the Mean Ghouls up to no good? When the most important monster in the country pays Monstacademy a visit, the threat of their school closing hangs over Trixie and her friends like a black cloud. To make matters even worse, there's a traitor in the ranks and Trixie and Colin have to rely on their own wits to save the day once again. Will a surprise visit from the Grand High Monster spell the end for Monstacademy?