Leonard's dad makes his debut in The Big Bang Theory's Season 9 finale Thursday, and his appearance sets off a series of events that could disrupt the natural order of the entire geek squad.

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Dr. Alfred Hofstadter (TV veteran Judd Hirsch) comes to witness Leonard and Penny's re-do of their wedding vows, and while it's Leonard's big day, his dad might be the one who ends up getting lucky... with Sheldon's mom!

At first, loathing Leonard's mom Beverly (Christine Baranski) seems to be all Leonard's psychiatrist dad and Sheldon's religious mom Mary (Laurie Metcalf) have in common. But by the time the gang gathers for the reception dinner, Mary and Alfred are thick as thieves, and skulk off together for a nightcap. [Insert live-audience "woo" sound here.]

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