Before Halloween, Team Scorpion must infiltrate a deep, vast cave system in order to prevent a bat population from destroying the North America ecosystem. Also, the team does costumes to stage photos from Halloween's past for Walter's upcoming immigration inspection.

When Team Scorpion travels to Ireland for Megan's one-year memorial, Walter discovers that a nearby lake's natural disaster has created a poisonous cloud that could kill everyone in the village. Also, Cabe and Toby try to alleviate the tension between Tim and Walter.