User blog comment:AwesomeIsMyName/The Last Episodes of Mystery Incorporated Discussion/@comment-14469071-20130404114234

Wikipedia has the descriptions for the next 2 episodes, and althoguh they are not official they sure sound so...

Gates of Gloom - Both the Mystery, Inc. gang and Professor Pericles attempt to unlock the way to a system of caverns, where the cursed treasure  and evil entity await.

Through the Curtain - The gang pursues Professor Pericles through the caverns.

Since it's part of three part finale, it's good that we don't know the last one's description, it would kinda spoil the previous one.