The Riddler is back, just in time for King Boris's visit, and the Queen of Freedom may never survive! The Bat-rewatch does "A Riddle a Day Keeps the Riddler Away"/"When the Rat's Away the Mice Will Play."
In the Batcave, Batman finds Robin fiddling around with the atomic pile (yeeeeeeeep) because he thought he heard some rumbling and wants to make sure there isn’t anything wrong (YEEEEEEEP!). Batman pulls him off that to help solve the last riddle: a woman in love is like a welder because they both carry a torch. The bomb is in the torch, placed in the replica when King Boris was Riddler’s prisoner. (They show a rather appalling lack of concern for the possibility of their friggin atomic pile malfunctioning, but whatever...)