Marieve Herington

Claire has overdone it in Halloweens past and the neighbors definitely don't share her enthusiasm, so this year she's being forced to tone it down and keep it "kid-friendly," and Phil has an idea to hold an open house on Halloween night. Meanwhile, Mitch and Cam host a costume party while contending with Lily wondering who her real mom is, and Gloria's pregnancy hormones are on overdrive, making her even more hotheaded than usual.

When Lily refuses to believe that Jenkins, Marshall's attractive female colleague, kissed him, Marshall insists Jenkins tell Lily what really happened. Meanwhile, Robin finds out that a drinking game has been made up about her.

The gang tries to help Ted when the perfect girl is finally available, and Marshall takes action when he finds a letter written by his younger self.

When Zoey enrolls in Ted's class, she manipulates his students into taking her side in the fight to save the building being torn down to make room for his project. Meanwhile, Marshall and Lily spot Robin marching in the Halloween parade of shame.

When Barney finally meets his father (guest star John Lithgow), he's surprised to learn how his life turned out. Meanwhile, the gang points out each other's "gaps," something they should know by adulthood, but never learned.