Mort Goldman, whilst searching for a bathroom, finds Stewie's room - and is transported by Stewie's time machine. On a desperate urge to get him back, Stewie and Brian find him on the day of September 1st, 1939 - the day Poland was attacked by the Nazi's. Trying to get back to the present day, Stewie realizes the supply of Uranium used to power the machine is empty - and they have to go to Berlin to find some more.
Stewie: Okay, if everything worked properly, this should be the exact time and place that Mort was sent to. Now, we've just got to figure out where we are.