Rei/Sailor Mars and her mysterious ally are both on a dangerous quest together to find a part of the lost past that could hold a key to the future. But the forces of evil are in hot pursuit and now it's up to their friends the Mystery Shack Investigators, Sailor Senchi, and M.A.S.K. to find them before they do.

It is near Valentine's Day and Mabel and Adam are about to celebrate but unfortunately someone wants to celebrate early, and both are the guests of honor. It's a Valentine's Day party they both better hope they get out of alive, or it will be their last.

Three MASK agents along with an agent of VENOM must work together to survive a strange dangerous land from an unknown enemy with more venomous motives.

The evil Gideon continues his hunt for the lost Grimores, one is hidden in a old military base in Gravity Falls. Ami Mitzuno and someone from her past Ryo Urawa are both forced into the hunt. But both are venturing into unholy ground for unfortunately both aren't alone, for evil from the past doesn't rest.

The neighborhoods of the Falls are in the grip of a faceless terror that uses a mysterious sent to keep their stranglehold of fear, unless Dipper and a mysterious ally can break their grip.