And the cave he's in now, um, then, is the future Batcave? The cave paintings are a message?
With his luck, it'll be Harley who'll figure it out...
Harley: WAIT A MINUTE!!! Maybe... Maybe he painted these!!!
Selina: Sigh... Harley, we've established that these were made in the Stone Age!
Harley: But... Instead of being killed, maybe he got sent back in time! And... And, he painted these so we'd find them, as a message! If we can work out exactly when these were painted, we can get some of those time travel guys, go to the right time in this very place, and rescue him!
Selina: Harley... You're not crazy, any more. Remember?