Pen Pal is the 17th short of Foster's Home for Imaginary Friends.
Bloo notices that Eduardo is writing a letter to someone, and as Bloo asks who he's writing to, Eduardo states it's his pen pal from England. Without any evidence Bloo immediately deduces that pen pal is the Queen of England, and gets excited over the idea that if he becomes friends with her, then she'll do something grand for Bloo like make him the King of France.
Bloo searches for a pen to write his letter to the "Queen", and Mac suggests that he use Nancy, a British-accented female imaginary friend who has a pen for a body. Bloo turns down Nancy, claiming she'll only make the letter too boring. Nancy storms off in a bad mood.
After writing the letter with a regular pen and creating a huge care package filled with lots of unseen gifts inside, Bloo then asks Eduardo to deliver the package to the pen pal, until it turns out that Eduardo's pen pal was Nancy all along. Nancy smugly rejects Bloo's care package as being "too boring".