Plague of Madness is the seventh episode of Primal and the seventh episode of the first season.
A sick and unstoppable sauropod has set its sights on two new prey.
In a tranquil forested valley, an Argentinosaurus herd goes about their daily business; feeding on vegetation, drinking from a watering hole, tending to their nests, and generally enjoying the peace and quiet. Suddenly, one individual has a strange encounter with a deathly ill Parasaurolophus that seems to be running about randomly as its body visibly rots. The Argentinosaurus ignores the creature until it bites it in the leg; it then kicks the hadrosaur into a tree, causing it to thrash around impotently before dying.
The bitten Argentinosaurus inspects its wound before heading to the pond for a drink, unaware that the Parasaurolophus was able to transmit a terrible infection with its bite. In the time it takes for the Argentinosaurus to reach the pond, its gait becomes lethargic and its flesh begins to decay at unnatural speeds; it drinks a large quantity of water in an apparent effort to soothe itself, but this only makes it vomit a torrent of blood into the pond (to the visible concern of its herdmates). Pustules begin bubbling over its skin and its eyes gain the same appearance as the infected hadrosaur's; a sign of the plague fully overtaking its body and mind.
Chaos erupts in the valley as the infected Argentinosaurus goes on a rampage, trampling nests and shoving the surprised parents aside. One member of the herd attempts to confront the infected one and gets its throat ripped out for the trouble; the plagued sauropod proceeds to massacre the rest of its former kin with bites, stomps, and tail whips. The last of the herd makes a run for it, but the infected one easily catches up and body-slams it into a tree, fatally impaling it on the pointed branches as its corrupted herdmate catches its breath amid the carnage.
Some time later, Spear and Fang are traveling through the same forest when they stumble across the slaughter. Both are visibly disturbed by the many corpses strewn about, untouched by scavengers or even flies; Fang sniffs one body and immediately recoils in apparent fright. As they take in the gruesome sight, the perpetrator rises up behind them and roars fiercely. Recognizing that they can't defeat it in battle, Spear and Fang flee through the woods with the infected sauropod in hot pursuit; they eventually lose it by jumping into a ravine, causing the beast to crash at the bottom and knock itself out.
Visibly panting for breath after their close call, Spear and Fang retire to a small cave on the cliffs to collect themselves and rest. While sleeping, Spear has a nightmare of the Plague Monster biting him, ripping a huge chunk out of his torso as the rest begins to decay. Spear can only look on in horror as his hands and face rot before his very eyes until a roar draws his attention; Fang has also been bitten, losing a large piece of her back before her flesh rapidly begins melting off of her bones.
Spear jolts awake from his nightmare and wanders out of the cave to check on the beast; it's exactly where he last saw it, seemingly dead. He debates their next course of action with Fang, eventually convincing her to sneak down the cliff and escape through the ravine On their way down, Spear disturbs a small rock that falls next to the infected Argentinosaurus; when it doesn't react, he takes this as proof that it is dormant before continuing his descent. After descending to ground level and sidling around the sauropod (also getting an unpleasantly close look at its putrefying skin), the duo are finally clear to tiptoe away...
...at which point the Argentinosaurus gets up and resumes the chase as its quarry flee through the ravine. Despite several close calls, Spear and Fang manage to escape through a crevasse too narrow for their foe to pass through, leaving it impotently roaring and flailing its neck as they descend into a volcanic plain. The open calderas begin erupting shortly afterwards, burning both Spear and Fang with stray embers as the ground cracks beneath the latter's weight. Realizing the instability of the area, Spear and Fang decide to double back and find another way out, just in time for the infected Argentinosaurus to literally tear itself out of the crevasse, leaving much of its rib cage exposed as it goes after the duo once more.
With no other option, Spear and Fang rush through the lava field, getting the Plague Monster's neck burnt by a lava geyser as it follows. This enrages it to the point of leaping towards them, causing it to break through the rocky crust and fall into the lava below. Spear and Fang ride the resulting wave to safer ground and resume running as the beast re-emerges, visibly burnt but no less determined to kill them. Unable to fully climb out of the molten rock, it instead dives back into it and erupts through the ground in front of its quarry; Spear and Fang land on its back, with the former being stunned at the glimpse of pure emptiness within the Argentinosaurus's body. Using its neck as a ramp, they are able to make it to the other side of the geyser field unharmed; the plagued sauropod tries to pursue, but its next attempt to jump out of the lava simply sends it falling back in.
Safe on the opposite cliffs, Spear and Fang watch in shock and morbid fascination as the infected Argentinosaurus once again emerges, further efforts to escape being stymied by the loss of its forelimbs to the lava. Fully trapped and bursting into flames, it can only howl in pain and maddened fury before finally collapsing and disintegrating into ashes. A single ember from the burnt Argentinosaurus lands on Spear's palm as he and Fang look over the lava field, seemingly contemplating the sauropod's gruesome end.
- In most of its scenes, the infected Parasaurolophus has an intact right hand and a skeletonized left hand. However, when it bites the Argentinosaurus, its right hand has rotted to the bone instead.
- As the infected Parasaurolophus runs around, the Argentinosaur has four claws on it's front feet, while other scenes it has a single claw.
- When the infected Argentinosaurus gets up to begin pursuing Spear and Fang, it is shown to be directly behind them. However, the previous overhead shot of the massacred sauropods shows the body behind the duo as an ordinary, uninfected corpse.
- The infected Argentinosaurus size changes between many shots, sometimes from regular size to double its size.
- Although it is the seventh chronological episode, this particular episode aired early on April Fool's Day (April 1st), presumably in light of the COVID-19 pandemic.
- Due to the premature airing, it was unconfirmed if this was the sixth chronological episode in the series, as there was little to no connection between the episode and "Rage of the Ape-Men". Notably, Fang appeared to be alive and well despite apparently dying at the end of the fifth episode. It was soon confirmed to be the seventh episode, after "Scent of Prey" (which shows Fang's recovery).
- The protagonists' encounter with the uneaten corpses of sauropods mirrors a scene from the 2015 film Jurassic World. Moreover, the death of the infected sauropod by the volcano is possibly a reference to the infamous death of the Brachiosaurus from the film's sequel Jurassic World: Fallen Kingdom.
- The scene where the Argentinosaurus vomited a large amount of blood references a deleted scene of the 2016 Japanese kaiju film Shin Godzilla (Godzilla Resurgence). It also bears similarities to the infected humans as seen in the 2002 film 28 Days Later.
- The appearance of the infected hadrosaur out of the forest is likely a reference to the baby Parasaurolophus that disturbed the stalking Carnotaurus in the 2000 Disney movie Dinosaur.
- The second chase from the infected Argentinosaurus in a narrow canyon somewhat resembles the sauropod stampede from the 2005 film King Kong.
- This is the second episode of the series to carry an TV-MA rating due to its violent content, preceded by "Rage of the Ape-Men".
- This episode won the Primetime Emmy Award for Outstanding Animated Program 2021.