"The Joker's Flying Saucer"
Joker fills the city with planted stories of an imminent Martian invasion, while he starts building his own flying saucer. To do so, he first has to steal a supply of the very light metal beryllium. On his way out of the Metals Research Wing of the Wayne Foundation, he captures Batgirl and Alfred (whom he assumes is a mad scientist). Alfred is forced to build the craft, but he manages to include a flaw that forces them to land after just a quick tour of the solar system.
A timebomb placed in the Batmobile ends up exploding in the Batcave. It destroys almost everything in it, and flings Batman (unhurt) up onto the atomic pile.