It went like this:
Surreptitiously exploring Luthor Manor, Jimmy Olson and Chloe stumble across something even Lionel and Lex don't know: that the bust of Shakespeare in the library has a hinged head that when flipped open reveals a switch, which when thrown opens up a secret panel in the wall, revealing two tarnished old firehouse-type brass poles.
Being recklessly curious as always, Jimmy and Chloe slide down the poles to discover—
a.) The 1960s-era Batcave, with all the gear therein covered with dust and cobwebs but otherwise still fully operational, and
b.) While Aaron Ashmore looks pretty doofy in Adam West's old Batman suit, Allison Mack looks really hot in Burt Ward's old Robin suit.
Whereupon they hop into George Barris's original 1966 Batmobile and take off for the joyride to end all joyrides...
This is all your fault, Ben-El.