The Sleestak pay tribute with baskets of food to a huge barbarian named Malak. Sitting on a throne in his cavern, the torch-wielding angry brute rejects their offering, instead insisting upon gold, jewels, and meat. When Malak throws down the gifts, the Sleestak cringe in fear; the Leader admits they have none of what the self-proclaimed god demands. As the barbarian pulls out a huge stone block from a shaft set in the floor, the Leader begs him not to release the Hidden River's waters into their tunnels. The Leader desperately objects that the Sleestaks' eggs will die, but cruel Malak throws the switch that diverts the water into the Lost City. The barbarian announces the flow will cease only after proper tribute has been made; with the strike of a gong and the wave of his torch, he shoos the submissive Sleestak away.

Trodding through the flooding tunnels, the Sleestak gather furs and wicker baskets full of offerings. Along with Enik in the Library of Skulls (by the way, Enik, like Cha-Ka, is now deep bronze in color), the Sleestak Leader seeks a skull's guidance. With eyes pulsating slowly, the skull reveals that "a god is not a god that has the flesh and cravings of a human; as a human is appeased, so can Malak be." Noting that Enik too is threatened, the Leader asks the Altrusian to interpret the skull's wisdom. Enik explains that they should offer Malak only human-owned goods, whereupon the Leader interjects, "the Marshalls!"

Inside their temple abode, Uncle Jack nurses Holly, who is ill with a bad sore throat, with a bowl of juice. Will tries to cheer up Holly by suggesting she recall fond memories of the local malt shop back home, but his irritable sister tells him to buzz off. Uncle Jack pampers his niece with a home-made feather pillow, then leaves her to rest. He steps outside to reveal to Will that her bad fever is worsening, and that what she needs is antibiotics. Jack asks Will if they had salvaged their survival kit, but his nephew notes that all attempts to dive for it in the swamp had failed (funny, the Marshalls had a first-aid kit in "A Nice Day", which would now be lost in the rubble of their old cave). Well aware of the danger from LuLu at the swamp, Uncle Jack decides they must recover the waterproof pack to get the medicine.

Down at the swamp, Will shows his uncle the location of their submerged raft (wait a minute -- the original opening credits depicted their raft resting in the jungle, whereupon Grumpy immediately harassed them) and notes that the depth is no greater than 15 feet. Will distracts LuLu by waving a bamboo pole in her faces while Uncle Jack effortlessly salvages the survival kit; when Jack notifies his nephew of his success, LuLu chomps the bamboo pole out of Will's hands. Uncle Jack brings up additional gear, including large a backpack and a hand-axe; though even more remains, it must wait for another time, since their urgent mission has been accomplished.

As the moons rise in the early night sky, Enik and the Sleestak arrive outside the Temple. The Altrusian, holding a water jug, instructs the Sleestak Leader to attack once he douses the fires in the braziers. Enik extinguishes the outdoor flames, whereupon they storm the Temple's interior. His voice full of shock and fear, Cha-Ka asks Enik what he is doing, but as the Altrusian puts out the last fire, he only reveals that what they do is for the good of all. Holly jumps out of her bed to try and halt the looting, but Enik only repeats the vague motive for their acts. He and the Sleestak pillage the Marshall's goods, prompting Cha-Ka to trumpet a warning horn. Uncle Jack and Will hear the horn just as they arrive back at the Temple, where they repossess small bowls of crystals from Enik outside. Uncle Jack tackles the Altrusian while Will rushes in through the door to toss both bowls-full of crystals onto the floor. Though the crystals explode upon impact, the Sleestak continue on unfazed. The Sleestak exit with claw-fulls of booty while Will and Uncle Jack rush to Holly's side. After climbing back into bed, she explains that Enik led the assault; Cha-Ka adds that all valuables were stolen. Will and Jack rush outside, but the Sleestak are long gone. They realize that the survival kit was stolen, too, and Enik, who remained behind, refuses to reveal where it has been taken.

Enik would rather submit to imprisonment before leading the Marshalls to Malak. He explains that the Sleestak must pay tribute when the moons are all in line (did the Marshalls forget that this astronimical alignment triggers one of the pylons [The Pylon Express] to open a time doorway to 1970's Earth?). Will scoffs at the Altrusian's reference to the barbarian as a deity; Enik then informs them that Malak has diverted the underground river. He notes how the flooding will soon engulf the Temple in addition to the Lost City, but Uncle Jack decides they will find Malak on their own. Cha-Ka eagerly volunteers his keen vision and sense of smell to help locate Malak. As the men check up on Holly, Enik labels them fools, considering their sacrificed possessions trivial, but Uncle Jack insists they need to recover the medicine. Jack offers to shut off the hidden river when he retrieves their survival kit, if Enik will tend to Holly; considering the offer to be in his interest, the Altrusian dispassionately agrees.

Standing in the chest-deep flood-waters within their tunnels, the Sleestak hand goods down a line. Meanwhile, the Sleestak Leader and his troops bring the tribute to a toothache-suffering Malak. The Leader begs Malak to douse the his extra torches since their eyes cannot take the brightness, and once he does the booty is brought in. Malak is pleased with the improved tribute, but demands one final gift -- a slave. The Leader offers the servitude of his strongest warrior, but Malak desires a servant able to tend to him both day and night. The Sleestak promise to return with a suitable slave, and are dismissed with the clang of Malak's gong. Nearby in the jungle, the Marshall men and Cha-Ka hear the gong, and hide just in time as the Sleestak go charging past. Malak tries gnawing at a turkey thigh given with the tribute, but roars from the pain of his tooth and examines it with a decorated mirror.

Uncle Jack leads Will and Cha-Ka into Malak's abode, where he sarcastically comments about a "god" getting a toothache. The barbarian is amused to see them, it being 1000 moons since his last human contact. Jack announces they have come to gather their gear and leave, but Malak laughs in their faces as he releases a portcullis which blocks the the only exit. He insists his guests stay a while to provide him with entertainment. Jack looks for himself down the shaft where the hidden river courses; Malak notes that when the lever is reversed, the flow goes to the Great Swamp. Jack reminds the barbarian that both the flooded-out Sleestak and dried-up swamp life will die, but Malak is more concerned with the origin in time of his intelligent visitors. Jack tries to throw the switch to restore the river to its natural course, but the barbarian proclaims that he is in charge, disregarding the fate of the Sleestak. He becomes angry and readies his sword when Will attempts to gather their stolen items; both unarmed and outmatched, Uncle Jack advises Will that they heed to Malak's demand. When the barbarian is distracted by his toothache, Uncle Jack claims himself to be a god; he digs into the survival kit, revealing a "magic" pain-killing pill. After some bogus hocus-pocus, Jack instructs the "Cro-Magnon god" how to use it; as Malak suspiciously tries the pill, Will quietly reminds his uncle that Holly desperately waits for the medicine.

Back at the Temple, Enik watches over Holly. Grumpy makes a ruckus in the jungle, but they are safe within the walls of their dwelling. The Altrusian adds wood to the fire, to keep his ill friend warm.

After a short time has past, the "magic" pill worked; the barbarian's pain has ceased. He thanks Uncle Jack but remains unconvinced that the clever engineer is a deity. The Marshalls try dismissing themselves, but Malak demands more miracles.

Huddled down to whisper their plans, Uncle Jack hands the medicine to Cha-Ka. He will hold the barbarian's attention while Will boosts Cha-Ka to the barred window to escape. Malak impatiently whacks his gong, demanding to be entertained. Jack amuses his captor by creating fire with matches; he then (not so discreetly) dunks the rest in an open water jug before handing them to Malak to try. Cha-Ka predictably bungles his escape by knocking down a clanky metal urn; Malak is angered by the ill-fated deception, and remains unimpressed by Jack's powers. Uncle Jack next claims the ability to make heatless light, which Malak considers impossible.

Outside the Temple, the Sleestak approach Enik, where they inform him of his fate as Malak's slave. The Altrusian reminds the Leader how they need his intelligence, not to mention his daylight-tolerant eyes, and suggests they use the Marshalls instead. The Leader asks Enik to lure them outside, but he explains that they are already with Malak to recover their gear. The Sleestak head back to the barbarian's lodging to free themselves of obligation.

As Uncle Jack finishes loading the survival kit's flashlight with batteries, Will hopes that they are not dead. With more bogus invocation, Jack calls forth light to flow out from his "sceptre", and -- viola! -- the beam shines in Malak's face. Following another incantation, and with a little help from the switch, the light just as miraculously disappears. The amazed barbarian demands the magic sceptre, whereupon Jack discreetly takes out its batteries behind his back. When the dim-witted barbarian tries to work the flashlight, its non-response befuddles him; Uncle Jack informs him that he has not the power. Malak first demands that the power be granted, then changes his request to a plea. Jack notes how instant light could be a powerful weapon against the Sleestak; the very creatures hop through the jungle toward them as he again discreetly reloads the batteries. Uncle Jack refuses to grant the power until Malak meets two demands: the first is to permanently shut off the hidden river. The barbarian eagerly agrees, throwing the switch and restoring the stone block, after which Uncle Jack, using a large mallet, breaks the switch to ensure the that the flow stays undiverted. Jack's second demand is to be able to leave with their goods, to which Malak also agrees. With a little mumbo-jumbo, Jack bestows the power upon the Cro-Magnon god; the barbarian is delighted when the flashlight works for him. Jack reminds Will and Cha-Ka that they must return to Holly quickly; just then the Sleestak arrive. Malak removes the door bolt and raises the portcullis up, and then to Jack's surprise, blows out the torches. The overzealous barbarian wishes to take full advantage of his newly acquired magic sceptre's light. The Marshalls sneak out with their goods just as the batteries in the flashlight fail; the Sleestak Leader, realizing Malak's foolish mortal status, instructs his warriors to reclaim their tribute. Uncle Jack reaches back through the open door to cut the portcullis' rope with the barbarian's sword; the gate then falls, locking Malak in with the revolting Sleestak.

The next morning in the Lost City, the Sleestak return with their repossessed goods. They rejoice in their victory, holding arms high; happy music plays in the background. At the Temple, the Marshall family is also pleased to see that Holly's temperature and appetite are back to normal. While Holly devours her meal, Cha-Ka feasts on a soap bar! Will asks the Paku where he got it, to which Cha-Ka replies "survival kit!". Holly informs her hairy little friend that the soap is meant for bathing, but Cha-Ka prefers his own use of the bar, and continues on munching happily.