It Only Comes Out At Night
Jack Tenrec is paying the City in the Sea a visit in the dead of night, accompanied by dependable compadre Kirgo. The city has started operations to pump fresh water into the city, and Kirgo tells Jack that there are some suspicious goings-on surrounding it; even if the rumours mean nothing, Jack still wants to check it out, doubtful that Scharnhorst will have achieved such a feat without some major health and safety risk. Kirgo isn't too concerned about it and is more worried about them both wasting their time than anything else.
Suddenly, something wraps around Kirgo's chest and drags him away!
Jack only turns around in time to see his friend's flashlight shattered upon the ground. Without so much as prior warning, he's soon set upon by a humongous beast which tries to crush him with its huge tail with relentless ferocity. He tries to take aim with his gun but is whipped aside, getting a stack of pipes dropped upon him before he can get up. After quite some time has passed, he manages to free himself...
But Kirgo is nowhere to be found.
The next morning, he naturally heads to the city council and demands Scharnhorst shut down operations at the docks until the area is investigated, stricken with rage and grief over the disappearance of his friend. The council members remain unfazed - the broken flashlight doesn't seem like terribly convincing evidence to them, and since Kirgo, the city's ferryman is now missing, there's no one to bring in fresh water from the mainland, so they much prefer the option of hurrying up and finishing the project.
Jack returns to the docks and is visited by Hannah, who shares her condolences for his loss. Wasting no time with grief, he decides to enlist her help in investigating the area themselves.
Hannah explains about the rumours - apparently some of the moles working on the project have vanished without a trace, and anyone who complains about it to Scharnhorst is reassigned. One of the city guards spies the two snooping around the area and begins following them...
But, whoops, he's ambushed by the monster
and taken away, his mangled radio the only remains that Jack and Hannah find.
Hearing the slither's roar, they delve deeper into the excavation site. Hannah marvels over how the moles have burrowed through concrete and steel in such short time, but they soon discover why - acid drums are littered around the site in a haphazard manner, some of them leaking and causing the ground around it to rot and collapse, potentially risking the whole city to come tumbling down. He had told Scharnhorst explicitly not to use this material, but it doesn't surprise him one bit that she ignored his recommendations.
What is a surprise to them is the beast crawling its way out of the hole in the ground! It stomps away, embedding footprints into the hard floor, with Jack and Hannah in hot pursuit. It leads them into what was once a marketplace but is now no more than a sunken hall; Jack is about to give a history lesson on what happened to the place when the creature whips them off the platform and into the water below!
Jack swims back to the surface just fine, but Hannah was rendered unconscious by the fall and is trapped by seaweed! He manages to free her and drag her to safety, and all is well. The next day they return to the council, bring a piece of evidence back with them - a slab with the slither's footprint embedded in it. They request a few days to explore the area themselves, believing the creature to be lost and confused, but Scharnhorst, sick of Jack's persistence about the subject, destroys the slab and orders them out.
Come nightfall, Jack prepares for another expedition, but not before contacting Mustapha, requesting his Cadillac and some hunting gear, explaining that he's tracking a whiptail. Mustapha and Hannah believe that whiptails are mythical creatures that are only works of fiction, but Jack is insulted by that remark - do they think a myth ate Kirgo?
The two enter the excavation site once more, Hannah posing the question of how they're even going to find their slither. Jack just suggests they follow their instincts, but when the site is immeasurably huge, that's going to be tough.
Another issue is that the deeper they go,
the darker it gets, and that's going to be a serious nuisance when they need to
find their way home. Hannah stumbles across a circuit breaker which activates
the lights, but even that doesn't help much, the lights having either died out
or are too dim to see much in.
And, of course, the structures and walkways are about as stable as a stack of cards, as Jack nearly finds out when the ground crumbles beneath him on a narrow bridge. Hannah pulls him to safety and they continue venturing forth. They soon find themselves in a pitch black room, so naturally the first thing on their minds is finding a light switch.
OH NO TURN IT OFF TURN IT OFF
It was actually just a squirrel caught in the light. Oh, light and shadow horrors! They discover this room is also filled with stinking toxic waste, which has leaked so much the floor has practically turned to mush...
Which is just what you want to know when the whiptail appears again! It smacks Jack's gun away, but they back away and leave it be, watching it sniffing the toxic waste - it believes the scent belongs to another creature, hence why it is continuously drawn to the site. Mercifully, Mustapha arrives on the scene in the Cadillac, giving the two a chance to flee to a safe distance.
The most important piece of kit Mustapha brought with him (besides a mighty fine set of wheels by General Motors) is the snare gun, allowing them to, in theory, trap the beast, though in practise probably just stun it for a while. Hannah concocts a plan - they drive around the site with a toxic waste barrel in the back seat to leave a scent, and they'll snare the thing on site. They do this for hours with no success and Mustapha is getting mighty sick of it...
Which is precisely when he's grabbed by the creature's tail and dragged away.
Jack is indubitably angered by this turn of events and isn't going to let this four legged freakasaurus take away another one of his friends, so he gives chase. They only have shadows and brief glimpses to follow it by, but that's enough for Jack to stay on its trail and pull off Dukes of Hazzard style stunts in the process.
The whiptail ducks into an alleyway, but rather than squeezing his set of wheels through there, Jack just drives over a friggin' roof.
Then, out of nowhere, we're treated to a young couple out and about on a romantic midnight walk. Needless to say, this tender moment is spoiled by having a car slide into their path and drive off. 
The whiptail's not far, but they witness it wading through the canal back to its den, and Jack claims he knows a shortcut. Across a rickety old rope bridge, no less. Would it surprise anyone if I said the bridge begins unravelling just as they're across the middle of it? Jack pulls Hannah close to his side to balance the weight and that gives them enough time to make it across before the whole thing crumbles to bits.
After some more bumpy rides, they make their way into a sewer tunnel, noticing telltale signs of a slither having set up a den nearby. They hear a quiet squealing noise from a mound of rubble and shine some light on the matter - it's a baby whiptail! Bundling it into the Cadillac, Jack believes it must've hatched while the mother was gathering food and wandered from the nest, but that reminds them... if the whiptail was gathering food, then that means... Mustapha? Oh no!
They drive a little further and soon find the whiptail and its nest, blasting it with the snare gun at the best available opportunity. While the beast experiences disco fever, they hear noises from another mound of rubble - it's Mustapha, Kirgo and the city guard! Alive and well, if in unpleasant condition, Jack tries to free them but finds the rubble to be far too sturdy to shift.
Hannah takes a look at the nest, noticing a large bundle of eggs, clearly their friends are being caught in storage until the eggs hatch - but they're not a healthy shade. Leaking toxic waste barrels litter the area, and from the looks of it the eggs have barely developed, which is bad news since the creature is unlikely to leave until its offspring hatch. This is bad news. And there's worse news - Captain Noc and the city guards are approaching, planning to shoot the whiptail on sight!
Jack tries to release the whiptail, but Hannah reminds him that it won't move until its eggs have hatched, so they have no other choice - brandishing a steel pool, she reluctantly shatters all of the eggs, the mother whiptail squealing in anguish at this sight. Jack reassures it, reclaiming the hatchling from his car and showing it to the mother, and both slithers are quite happy to be reunited.
The guards prepare to shoot, so while Hannah sets about releasing the whiptail from its snare net, Jack lunges at the guards, distracting them just long enough for the creature to escape into the canal with its child in tow. Jack bids farewell to the mother beast, wishing it well in its future endeavours.
The next day, Jack is in a good mood - Mustapha and Kirgo are safe and he stopped the slither threat without too much hassle. Hannah shows up and informs him about her meeting with the council - thanks to their good job getting rid of the whiptail and exposing the health and safety risks, the council have now promised to keep a closer eye on the water pumping project, especially when it comes to waste disposal. Jack is pleased, but is always wary that future generations will make the same mistakes they do now, but Kirgo reminds him there's always someone out there willing to do the right thing, and for now, that's their job.
Marty Isenberg and Robert Skir were the duo who penned this episode. They've worked together on a lot of cartoons - Batman: The Animated Series, X-Men, The Mask, Beast Machines and more.
For the first part, the animation in this episode is rather mediocre and the characters are less detailed than usual, for instance. Once Mustapha shows up with the Cadillac, however, the ensuing car sequences are probably some of the best in the series, with some noticeably smooth animation.
 I admit this part it's a little hard to tell where the action is taking place - the characters still refer to it all as an excavation site (Jack says there are at least three of them for the water pumping project), but a lot of it just looks like the regular cityscape. They're never around when the moles are working, and being an excavation site it's not implied people have actually lived there in a long time. So... what, these two just decided to go for a walk in a dangerous and desolate construction site that is reportedly easy to get lost in just so they could make mushy faces at each other?
What's a squirrel doing underground, for that matter? (addendum: I have just learnt of such a thing called a ground squirrel. Fascinating! I still want to know what's the dealio with this one, though.)