|This article's title is conjectural and may be subject to change as new information becomes available.|
The Red Sky seasons or Red Sky years is a popular term amongst fans referring to the final 3 seasons of the 1987 animated television series. At the time, cartoons such as Batman: The Animated Series and X-Men: The Animated Series (known for its darker tone) had begun to influence action-adventure cartoons, which is ironic considering that the cartoon is partially based on the original Teenage Mutant Ninja Turtles comics.
The show was retooled into a more action-based series. Among the changes, the tone of the show became darker and more serious, the art style changed significantly, with the sky constantly portrayed as red, instead of the blue (hence the term). The Turtles' demeanor evolved into a more serious and determined one than in prior seasons, and they devoted most of their time to tracking down villains and many of the previous notable characters such as Irma were written out of the show. There was also a new theme song, which is a remixed version of the original Teenage Mutant Ninja Turtles theme song and included clips from the first live-action film. Despite its tone, the show is still considered a continuation of the 1987 series.
Following the events of the Previous Season and picking up several weeks later, the Turtles try to track down Shredder and Krang. Meanwhile Shredder and Krang were forced to work out in the Hal of Science, until they can find a way back into Dimension X and retrieve the Technodrome. Their troubles compound when Drakus (alias Berserko), the former ally of Krang who designed the Technodrome, shows up for revenge. When Krang is taken hostage by Drakus, the Shredder persuade the Turtles to rescue him before he’ll blow up the Channel 6 building. The Turtles have to save Krang from Berserko, and get him back to Shredder before the Channel 6 Building is blown up with everyone inside it. After they successfully rescue Krang, the Turtles didn't made it on time and Shredder blew up the Channel 6 building.
Enraged at the seeming demise of their friends, the Turtles attack Shredder and company, defeating them and tie them up. Unfortunately they have to stop Berserko and his war machine, the Annihilator Robot that is attacking Manhattan forcing them to leave Shredder behind. After successfully defeating Berserko, the Turtles escape with the new villain in tow as the Annihilator plunges into the Hudson River and explodes upon impact. The Shredder manages to escaped and the Turtles are grief-stricken over the loss of their friends. However Splinter managed to rescue everyone inside the Channel 6 building. Because of this incident Burne blames the Turtles for the building's destruction, as well as other disasters caused by Shredder and Krang, as he declares the Turtles as public enemy number one.
Throughout the season, the turtles relentlessly pursue their arch enemies in an effort to put an end to their schemes once and for all. Eventually, Shredder and Krang, along with Bebop and Rocksteady, build a new portal into Dimension X and reclaim the Technodrome, although the Turtles manage to track them down with the help of Gargon, a mutated resident of Dimension X who was being held prisoner by Shredder and Krang. At the end of season 8, the TMNT finally banish Shredder, Krang, Bebop, and Rocksteady to Dimension X by destroying the Technodrome's engines and trans-dimensional portal, preventing them from returning to Earth.
With the Turtles' enemies Shredder and Krang stuck in Dimension X with no way out following the destruction of the Technodrome. An evil alien warlord from Dimension X named Lord Dregg, appeared and takes their place as the main villain of the series. He then reveals himself to humanity, not as a warlord but as a philanthropic savior and begins a propaganda campaign against the Turtles, turning the general population against them and in favor of him and his forces.
The Turtles also gain a new ally in the form of Carter, a brash African American male who initially sought out Master Splinter for training in ninjutsu, but is eventually exposed to mutagen and contracts an incurable mutation disease. Aditionally, the Turtles began to suffer from unstable mutations that temporarily transformed them into monstrous hulks with diminished intelligence. The Turtles will now have to deal with their mutations and stopping Dregg's forces from destroying earth.
In the end of the season, Dregg receives the Vortex Crystal, which can warp space and time and allow him to summon his scattered armada. He needs a great deal of power for this, but knows he can trick humans into giving him what he needs. As the first part of his plan, he "gives" the crystal to Earth's people, claiming it will create unlimited clean energy, in order to make them plug it into a power plant. Suddenly a massive ship appears overhead. A large alien named Doomquest appears, and claims the Vortex Crystal belongs to him, and traps Dregg's ship in the hangar of his own. The crystal is then falls into the hands of the Turtles, Carter and April but lieutenant Drakonis chases after them and manages to take the crystal from them.
As the crystal is energized by Drakonis, the dimensional barriers start to rupture, strange winged creatures emerge from a dimensional vortex with the entire city is being invaded by monsters from another dimension. The Turtles set out to stop the extra-dimensional monsters, but their increased aggression and diminished intelligence from the unstable mutations, causes them to fight each other as well. They arrive at Doomquest's ship and manage to find the crystal with relatively little trouble. Unfortunately, they also find Doomquest waiting for them, and immediately all four of them mutate again. Carter manages to grab the crystal as the Turtles begin fighting amongst themselves, and uses it to turn them back to normal. Donatello then tries to keep the Dark Realm from overrunning their own dimension, but the Turtles find themselves pinned down and unable to escape. Leonardo throws the Vortex crystal against the side of Doomquest's ship, shattering it and sending Doomquest and all his monsters back into the Dark Realm.
Dregg and his followers then flee the planet with him eventually outed as a villain, as April was able to successfully expose his true intentions to the world, and the Turtles are finally hailed as heroes within the city. Donatello spots pieces of fragments from the crystal, which they might be able to use to stabilize their mutations.
The Turtles attempt to use the pieces of the Vortex Crystal from the previous season to cure themselves of their out-of-control mutations. Meanwhile on a far-out planet, Dregg found out that the Turtles had discovered the crystal has the power to stabilize their mutations, but due to low readings, they have only a few fragments. With a tiny piece Dregg can genetically recreate another crystal then complete his Vortex Transporter. His new assistant Mung reports that the Dreggnaught is ready for departure, he even shows him a container filled with Microbot, which can build whatever Dregg desires in only hours. Because of his devious intelligence and skill in technology, Dregg offers Mung a position as his second-in-command.
In the near-end of the season with the Turtles finally cured from their mutations, they now have to face Dregg once and for all. Mung then encounters Shredder and Krang, who are still stranded in Dimension X. They told him that they had battled the Turtles for years, but even though Shredder claimed to have destroyed them, Mung knew that he was lying. Soon afterward, Mung returns to Dregg's ship and informs him of their encounter, and Dregg decides to bring both Shredder and Krang back from Dimension X to help him fight the Turtles. However, the pair immediately rebel against Dregg and leave, continuing on where they left off before they were banished at the end of season 8.
Back on Earth, Shredder and Krang kidnap April and do battle with the Turtles once more, although they are all soon transported back to Dregg's lair. The Turtles initially have the upper hand in the fight, but Shredder and Krang are able to subdue them after reluctantly agreeing to work with Dregg. As he prepares to drain the Turtles of their life energies, Shredder and Krang betray Lord Dregg and force him onto one of the operating tables, intending to drain both him and the Turtles of their power. Dregg, however, manages to escape and uses his microbots to capture Shredder and Krang.
Although he successfully drains the Turtles and Krang of their life energies, Shredder breaks free before Dregg is able to take anything from him. Shredder spends the next two episodes finding a way to heal Krang and dispose of Dregg so that they may take control of his armies and conquer the Earth, but in the ensuing confrontation they are permanently transported back to Dimension X. Carter also bids farewell to the Turtles as he travels to the future to look for a cure for his mutation.
In the final episode of the series, Michelangelo and Donatello travel to Dimension X to retrieve Krang's mechanical body from the (now abandoned and completely destroyed) Technodrome, which is sitting on a hill standing upright (whereas at the end of season 8, an alien plant had dragged it down into a deep pit), suggesting that Shredder and Krang initially tried to repair the Technodrome before declaring it a lost cause. Shredder, Krang, Bebop, and Rocksteady are nowhere to be seen. The Turtles eventually find Krang's suit and use it in a final confrontation with Dregg, which ends with the Turtles banishing Dregg to Dimension X. Splinter congratulates the Turtles on their victory and, now that all of their enemies have been vanquished, states that he has nothing more to teach them, calling them his equals.
The changes of the Red Sky Seasons were met with different reactions from fans. Some fans such as James Rolfe (a.k.a The Angry Video Game Nerd) praised the Red Sky seasons, finding them to be a refreshing take from the previous seasons, while some fans despised it, with them calling it "try hard" and the fact that it has jarring changes and removed several characters. However due to the series airing at the time where the franchise has declined in popularity and it being overshadowed by other popular shows such as the Mighty Morphin Power Rangers , the seasons have long been forgotten by most fans. 
- Season 9 was the last season to have David Wise involved as a writer, although he was a ghostwriter for the majority of Season 10.
- The seasons aired on CBS as part of their Action Zone programming block, which ran from 1994-1996 aired alongside with Wild C.A.T.s and Skeleton Warriors.
- The aforementioned shows were canceled around the same time that the Action Zone brand was officially retired, although TMNT retained the name in its credit sequence until the end of its run two years later. 
- Dregg was created due to the fact that Power Rangers was changing their villain at the time. 
- With the exception of the Cyber-Suits, no character or concepts that were introduced during the Red Sky seasons were ever made into a toy by Playmates.
- The Turtles' unstable mutations was a recycled concept conceived for the scrapped fourth live-action movie.
- A "talking" Electronic LCD game, Teenage Mutant Ninja Turtles: Dimension X Assault based on the season was released by Tiger Electronics in 1995.