Marshall and Lily's plan to buy an apartment is hampered by mistakes and Lily's hidden debt.

Barney's efforts to hook up with women are being thwarted by a mysterious woman from his past. Barney and the gang must sift though his many past flings in an attempt to find out who she is.

Lily comes up with a perfect plan to break up the relationship between Barney and Robin when they have a rough patch, but she needs help from Robin's famous friend, Alan Thicke, in order to make it work.

In an attempt to deliver a devastating final slap to Barney, Marshall explains how he mastered the Slap of A Million Exploding Suns.