The Season 2 episode "Fountain" is a Christmas Episode that ends on a happy note...but the majority of its story is actually quite harrowing. Fi is trapped going backwards in time in increasingly younger versions of herself but with her present-day mind, eventually leading to her as baby, unable to speak. She does eventually wake up in the modern day with everything back to normal, but until that happens there's nothing to suggest she won't just keep un-aging until she ceases to exist.
