S11, E19: Walden reconnects with Kate and is sure she's "The One" – that is, until he meets Vivian (guest star MILA KUNIS – That '70s Show, Black Swan), a beautiful, free-spirited world traveler who shows up on his doorstep. Feeling an instant connection with Vivian, he soon becomes convinced that she's "The One." Meanwhile, Alan and Gretchen grow closer.