S4·E46 Forget Me Nuts

Beetlejuice loses his memory (he's beaned by a satellite), so Lydia takes him to see Dr. Zigmund Void. Void splits BJ's personality, and takes the clone and Lydia within BJ's body by way of a shrinking submersible. They have to find out what is causing BJ's inability to access his memory banks before time runs out and they return to normal size.