The episode begins with Nibbler showing signs of illness and rapidly losing his intelligence. The cause is traced to the same parasites that once inhabited Fry's body in the earlier episode "Parasites Lost," but now these worms have taken up residence in Nibbler's litter box. Unlike their previous effect on Fry, the worms are causing Nibbler's mental capacity to deteriorate, threatening to reduce him to the intelligence of a common Boston terrier.

Leela and the Planet Express crew try to treat Nibbler with medicine, but it fails because the worms in the litter box keep re-infecting him every time he uses it. They cannot simply change the litter because it is made from precious sand containing essential minerals from Nibbler's home planet's ancestral pooping ground, which he needs to survive.

Faced with this dilemma, the crew decides to shrink themselves down using Professor Farnsworth's shrinking ray to enter Nibbler's litter box and confront the parasites directly. They embark on a journey across the vast desert-like landscape of the litter box, which is inhabited by giant sandworms and other creatures, including dung beetles.

As they explore, they discover that the worms themselves are infested with tiny sub-parasites--mites--that are sucking their blood and weakening them. This infestation explains why the worms are harming Nibbler instead of enhancing his intelligence as they did with Fry.

Leela pleads with the Worm Queen to allow her to use the Glitter Chamber, a device that induces hallucinations to reveal deeper truths. Despite warnings about possible side effects, Leela insists on using it. Inside the chamber, she experiences vivid hallucinations that help her understand the situation more clearly.

Emerging from the Glitter Chamber, Leela explains that Nibbler was right about the worms' potential but overlooked the mite infestation. She declares that they must kill the mites to save Nibbler and restore the worms' beneficial effects.

The crew, along with the worms and dung beetles, engage in a coordinated effort to stomp out and kill the mites. This battle involves physically brushing themselves free of the mites and smashing them, effectively cleansing the worms of their parasites.

After the mites are eradicated, the worms regain their strength and intelligence-enhancing properties. Nibbler's mental capacity begins to recover, and the crew returns to normal size, escaping the litter box aboard Nibbler's ship.

In the final moments, Leela reflects on the experience and acknowledges that Nibbler's warnings were correct. The episode closes with Nibbler's intelligence restored, and the crew having saved their beloved pet from the parasitic threat.

At the end of Futurama Season 8, Episode 4, "Parasites Regained," the PlanEx crew successfully cures Nibbler of the intelligence-draining worms infesting him by discovering that the worms themselves are infected with tiny mites. After a battle to eliminate these mites, Nibbler's intelligence begins to recover, and Leela gains a new understanding of the complex parasitic ecosystem affecting her friend.

The episode's ending unfolds as follows:

After shrinking down to enter Nibbler's litter box--a vast desert-like environment--they confront the worms that have taken residence there and are causing Nibbler's rapid cognitive decline. The crew returns to normal size and escapes the litter box aboard Nibbler's ship. Leela pleads with Nibbler not to let the worms consume his consciousness, but his intelligence continues to fade.

Leela, Fry, and Bender then return to the Hall of Dung, a sacred place connected to Nibbler's species. There, Leela requests permission from the Worm Queen to use the Glitter Chamber, a device that induces hallucinations to reveal hidden truths. Despite warnings about side effects, the crew insists on proceeding.

Inside the Glitter Chamber, Leela experiences vivid hallucinations that allow her to perceive the intricate parasitic relationships described by Nibbler. Emerging from the chamber, she declares she now understands the situation fully and confirms that Nibbler was right about everything--except for one detail: the worms should be making Nibbler smarter, as they did when they inhabited Fry's body previously.

The Worm King questions why the worms aren't enhancing Nibbler's intelligence. Leela reveals that the worms are weakened because they are infested by sub-parasites--tiny mites that Zoidberg had earlier described as "cute." These mites are sucking the worms' blood, undermining their beneficial effects.

Leela insists that although she is an animal lover, they must draw the line and eliminate the mites to save Nibbler. The crew agrees, and after a vigorous effort, they kill the mites. Freed from the mites' parasitic drain, the worms join forces with the PlanEx crew and beetles to destroy the mites completely.

By the episode's conclusion, Nibbler's intelligence begins to recover, and the parasitic ecosystem within him is restored to a healthier balance. Leela gains a deeper appreciation of the complex layers of life and parasitism affecting her friend, while Fry and Bender support the effort to save Nibbler.

Thus, the main characters--Nibbler, Leela, Fry, and Bender--survive the ordeal with Nibbler's health and intellect restored, and the episode closes on a note of cautious optimism about the intricate biological relationships at play.

The episode "Parasites Regained" from Futurama Season 8 (Episode 4, 2023) does not have a post-credit scene. None of the available detailed episode reactions, reviews, or summaries mention any post-credit or after-credits scene following the episode's conclusion. The episode ends with the resolution of the parasite storyline involving Nibbler, and no additional scenes or stingers appear after the credits roll.

Nibbler falls ill because he is infested by the same kind of stomach worms that once inhabited Fry, but these worms are now weakened due to being infested themselves by tiny mites that suck their blood, which diminishes Nibbler's intelligence.

The crew shrinks down and enters Nibbler's litter box, which is a desert world inhabited by the worms, to investigate and combat the parasites threatening Nibbler's health and intelligence.

Leela leads the effort to save Nibbler, including convincing the worms to use the Glitter Chamber despite its side effects, and ultimately deciding to kill the mites infesting the worms to restore Nibbler's intelligence.

The mites are sub-parasites that weaken the worms, preventing them from making Nibbler smarter as they had done for Fry previously; eliminating these mites is crucial to saving Nibbler.

The episode revisits the concept of the stomach worms that once inhabited Fry, showing that these same worms now live inside Nibbler, but with a twist that they are weakened by mites, reversing the effect on intelligence compared to the original episode.

The Futurama Season 8, Episode 4 titled "Parasites Regained" is generally suitable for a family audience but contains some content that might be unsettling or objectionable for very young children or sensitive viewers.

Potentially upsetting aspects include:

• Scenes involving parasites and worms inside a litter box, which may be visually gross or disturbing to some children.
• The concept of shrinking down to explore a sandy, microscopic world inside animal waste, which involves bodily functions and might be considered icky.
• Some mild dark humor and references to intelligence loss due to parasitic infection.
• Brief moments of mild cartoon violence or conflict as the crew battles parasites.
• A few subtle adult jokes and cultural references typical of Futurama's style, which might go over children's heads but could be noted by sensitive viewers.

There is no strong language, graphic violence, or explicit content reported in this episode. The humor is mostly in line with the show's usual sci-fi satire and quirky adventure tone. Overall, it is more appropriate for older children and up, especially those familiar with Futurama's style, but parents of younger or sensitive children might want to preview it first due to the parasite and litter box themes.