Plot: Kimmy's invisible friend Mojo is real and robs jewelry stores. That's because he's actually Kimmy's father Lloyd Ventrix in an invisibility suit, and he's going to kidnap Kimmy if Batman can't find a way to stop him.
Hands down, this is one of the darkest, creepiest episodes of Batman: the Animated Series. Partly, that's due to the natural terror of an invisible villain, one who could be in the room RIGHT NOW. The animators pull out every trick they can to let you know where the man you can't see is, the dog that barks at nothing, the swinging gate that holds still a split second too long, panning the camera along a path, all things that you might see on any day, and they let your mind fill in the man who isn't there. That's before getting to the more overt stuff like the footprints in concrete, floating dolls, and Batman getting the stuffing knocked out of him.
But above and beyond the fantastic elements, the chilling part of this episode is the very real child abduction plot, along the lines of Be a Clown, but more so. Maybe that's because as a supervillain, the Joker always has an element of unreality to him, so his child abduction episode is a little more distant. On the other hand, most child abductions are committed by family members, so Lloyd is a villain that not only can exist in real life, but does. (And I appreciate that he IS Kimmy's father and not a stranger. If parents need to be needlessly scared someone is going to take their children, they should at least be scared of the right people).
Michael Gross captures the unsettling nature of Lloyd extremely well. Even at his nicest, when he's playing Mojo for Kimmy, there's a need to impress, to control Kimmy's emotions. When he grabs Helen's arm and shouts "Look at me!", we don't need to guess why she divorced him and got a restraining order (a good character moment: Kimmy is afraid of Lloyd when he's a stranger, but when he tells her he is her father, Kimmy is EVER MORE afraid of him). He even needs to impress Batman. He could just run away, or beat up Batman in silence, but he needs to gloat, to show Batman that he's the better man.
(Also creepy, Michael Gross was the father on Family Ties. Did the casting director for this show pick the nicest guy on every 80s sitcom to play the most disturbing villains? Is that how Bull ended up as Two-Face?)
The unsettling feeling is sustained by the music. For the Mojo scenes, we get a high pitched xylophone with a dread inducing cello underneath. This feeling is only made worse when we transition from "Land of the Perfect Day" music out of Peter and the Wolf for the Helen and Kimmy scenes back into the disturbing theme. In the finale, when Kimmy is talking to her new "imaginary" friend Batman, the cellos have been replaced with Batman's fanfare horns, so we know that everything is alright, unless you're a criminal.
For such a dark episode, it also contains some of the series funniest and silliest gags, as if the writers desperately needed to balance the show. The guard, biting his lip waiting to use the bathroom while Bruce Wayne changes, only to be smacked in the face when Batman dramatically emerges. The construction worker's "who me?" face when Batman yells at a seemingly empty room. The bum who, seeing Batman clinging to the top of an invisible car, turns to his friend and says, "I didn' know he could fly, too."
Oh yeah, the invisible car. That is very well pulled off. Not just the animation of Batman flying around the streets of Gotham, or letting scratches in the invisibility coating let the car appear slowly, but just the reveal that the car could turn invisible, this car that we've seen since the beginning, that's the same color as the un-activated invisibility suit, is a great way to both set up and surprise the audience while elevating the threat of the villain for the final confrontation.
Also, I have so much respect for how the writers show Batman is smart, rather than tell. He's immediately and constantly throwing smoke bombs and paint into the air to mark Ventrix, while in some shows, and I'm looking at you, Smallville, it takes the hero an hour to arrive at the same solution. Batman doesn't let a little thing like the impossibility of the situation stop him from kicking some ass, all leading to a fight under a leaking water tower and the most badass delivery of "Peek a boo" ever in Western civilization.
And finally, the episode introduces Lucius Fox, who was created in the comics to answer the question, "If Bruce Wayne pretends to be an idiot, how does Wayne Enterprises stay in business?" While Wayne owns the company, Lucius is the de facto head. Also, Lucius is the only one who know Bruce is not the idiot he pretends to be, though he probably does not suspect what Bruce actually does at night. In the Animated Series, Bruce is shown to be more active at the company, so Lucius is just second in command. That does leave the question of when does Bruce Wayne sleep, but maybe that's thinking too hard.