What a great episode!
[spoiler]Rose is my favorite companion so it was great seeing her again. The episode takes you through all the horrible things that happen when the doctor never meets Donna. The Doctor’s death beneath the Thames, Sarah Jane and Martha dying when the hospital goes to the moon, Gwen Cooper and Ianto Jones dying fighting the Aesmos systems and Captain Jack getting transported back to Soltaran all happen because Donna took one wrong turn in her life under the influence of an alien bug that has attached itself to her back. Rose, who has been pulled across dimensions, sends Donna back to that fateful turn with a message for the Doctor, “BAD WOLF”! Duh duh DUUUHHH![/spoiler]