Plot Synopsis
On April fools the Joker sets out to make fools of everyone in Gotham city using a garbage trawler boat doused in powerful chemicals, the fumes from which cause insanity.
Batman battles Joker on the barge and at Jokers lair (which is located in a centre for burning rubbish).

Highlights

Villians
Joker once again takes centre stage, with a plan disigned to spread chaos and confusion. He also enjoys showering his followers with an onslaught of puns at almost everything that occurs. I suppose when you agree to work for the Joker, you have to be ready to put up with that sort of thing (at least, if you dont want the joy buzzer of death!)
Joker makes use of some very cool steal playing cards (hidden up his sleeve) which he hurls at batman. It is when Batman catches one of these out of the air that Joker loses heart and attempts to flee (I dont blame him! Batman catching that card was VERY bad ass!)
Joker is great in this episode, especially during the final chase around the furnace. There is a sequence where Joker rides down a conveyor belt, sitting cross legged on top of the square that used to be Captain Clown (more on him in a minute!) Jokers blank, staring face with its huge, unmoving grin is really creepy, especially as the light and shadow pass over his face.

As well as two thugs (both wearing clown masks that contain gas masks) he also has the assistance of the mechanical Captain Clown. Captain Clown appears to be a large clown in a sea captains uniform (as you might expect!) but his blank staring face is a cover for his robotic workings. We can only assume that Joker stole this robot from some military research lab prior to his latest crime spree, an idea further supported by the submarine Joker has under the barge, powering it along. Batman smashing the captains head open and then crushing him in a car crusher is very satisfying, and made even better by Jokers appalled reaction-
“You killer Captain Clown…YOU KILLED CAPTAIN CLOWN! Just for that…”
*drops scrap metal on Batman from a crane*

Batcrap Crazy
The Joker delights in spreading mayhem, and here he gets plenty of it to enjoy! Horrific as his plans are, there is some method to his madness this time, as him and his thugs manage to make off with a great deal of money and valuables (including breaking open an armoured bank car that crashes into the river).
The promise of loot is obviously what is keeping the thugs in the Jokers employ, but Joker certainly enjoys profiting from his crime, showing there is some logic (all be it a twisted, immoral logic) to his actions. Of course, after the initial attack he shows no intent of stopping, so clearly there is never a chance of taking the money and running. Joker plans to keep spreading the gas until he is stopped. No doubt the two thugs are assuming their boss will have an escape plan for them all to get away with the money, but they are sorely mistaken!

The citizens of Gotham unwittingly partake of a lot of madness in this episode, as Jokers gas turns them into laughing maniacs. When even Alfred gets affected and starts running amok, it really puts the pressure on Batman to put a stop to the chaos. Great stuff!

Batman once again shows his absolute, unshakable, code against killing in the end by pulling the Joker back up when he is dangling over the flames (though Batman does tease that he might let Joker fall, this is clearly just to make Joker scared, not because he was ever really considering it.) Batman simply wont let people die! It is not even a code against killing, it is a code against letting others even die through your inaction when it is their own stupid fault and they clearly deserve it! It doesn't get much crazier then old bats…

GAS!
I bet Scarecrow wishes he could pull off a gas attack on this scale with his fear gas!
Joker uses gas on a VERY grand scale here, covering a huge area of Gotham (including the financial sector, leading to the cool stock market reading saying Joker stock is up 100%)
I also enjoyed seeing Summer Gleeson (Gotham's favourite reporter) doing her report in a gas mask and showing obvious distaste for the peoples insane behaviour.

Death Trap!
During their first battle Captain Clown spins Batman so he is disorientated and then slams him into a barrel, which Joker seals shut. Joker then stabs at the barrel with a long dagger (claiming mockingly to be providing air holes). Batmans supreme reflexes allow him to evade the thrusts, but then Joker has Captain Clown throw the barrel over board (once again Joker mocks Batmans peril, pretending to be upset to realise that the air holes are now water holes).
So Batman is sealed in a barrel with water coming in, at the bottom of the river!
Batman escapes by remote calling his bat boat and using its laser to cut him free. Since he is firing the laser blind, he comes pretty close to slicing himself in half, but he does manage to get free and surface.
What is great about this death trap is two things-
Firstly, it is set up convincingly. I never felt Joker was letting Batman off lightly here or was failing to kill him when he had the chance (as one might in a more cheesily set up death trap). The situation where by Batman is left alone by the villain to find a way out comes naturally, because he is thrown to the bottom of a river and so cant be observed. The whole situation comes off very convincingly.
Next, Batman holds his breath under water for quite a while, but its a believable length of time. Batman is incredibly in his abilities, but he is human. That is why it is great that they keep the length of time quite long, but still believable.
This is further enhanced when Batman is shown to be concentrating hard while in the barrel, using all of his self discipline to hold out until the bat boat arrives. It is nice to see that the hero is having to work hard to keep going under such circumstances, a detail a lesser cartoon would have neglected to include.

Holy Shitty Moments Batman!
The only things that I would take away from this episode is that it ends with Joker hanging over a fire pit and Batman saving him, the exact same ending with saw in ‘Christmas with the Joker’. Especially because the two episodes are so close together chronologically, this detracts from both of them. It is a cool concept that gives a good insight into Batman's own madness, but to do it twice in exactly the same way just does not live up the standards this show sets for itself.
My only other point is more of a question then a complaint…
Joker wears a helmet to protect himself from the gas, but what would it have done to him? He is already a laughing maniac, so can he be affected by his own chemicals? Possibly the brain damage this particular chemical caused (long term exposure causing permanent insanity) was something he just didn't want to risk.

Summary
This is a great episode. It is a real classic match up of Batman against The Joker. Nothing complex or fancy, just Joker spreading chaos, Batman trying to stop him. We get to really see these two classic enemies face off and do exactly what they both do best. This is what Batman is all about.

Final Score
4 out of 5

(I had to move this one to a new thread, due to an error in the previous title.
Wordweaver_three previously stated the following)

You know you're a Gotham City reporter when your standard equipment includes a gas mask.

Not a lot I can add to this, you pretty much hit all the key points of the episode. Though you did leave out the “does Batman have a sense of humor” sub-plot. At the beginning of the episode Alfred announced to Bruce that he “Drew him a bath”, when Bruce enters the bathroom in a robe only to find an empty bathtub, he questions his butler. Alfred explains (in his deadpan voice) “I said I drew you a bath sir.” and holds up a sketch of a bathtub “April Fools”. Bruce is clearly not amused. “Not a funny bone in his entire body” notes Alfred to himself.

When Alfred becomes affected by the joker gas, he smashes up several items in the mansion (presumably expensive antiques). At the end of the episode when he apologizes to Bruce for the damage, Bruce tells him it can come out of his pay. Looking rather downcast, Alfred replies, “Of course, sir”. As Bruce is walking away, he turns back and says “April Fools”.

The Joker also accused Batman of “finally telling a joke”, much to his delight.

So Batman does have a sense of humor, he just hides it very well.

Thanks Word Weaver, you just filled in the gaps pretty well there.
Batman not only wins the joking contest with Alfred at the end of the episode, but also gains a similar victory over the Joker.
Though Joker mocks Batman for making a joke, as I mentioned in the review Joker ends up at Batman's mercy with Batman pretending he might let him fall.
So he comes out on top in the April Fools battle against both Alfred and the Joker (though the trick on the Joker is a LOT crueler!)

Now this is an episode were the Joker really gets to shine!

Captain clown and the Joker's gleeful air-hole aplication are so wonderfully memorable.

“It looks like our air holes are also water holes”

Mark Hamill is da man!