Some TMNT stuff really isn't for little kids.
Prelude to Vengeance is a comic book produced for Free Comic Book Day, taking place shortly after the first issue of the Vengeance arc. A few of the scenes are new material, but much of the issue is composed of flashbacks taken directly from past issues along with narration by some of the characters, much like a television clip show.
Splinter meditates on his existence as both a ninja and a father, and how fatherhood has brought him immense joy and "immeasurable pain." He reflects on how his unyielding principles have led to Oroku Saki as a lifelong enemy, the death of his wife, the loss of his home, and finally the murder of himself and his teenage sons in a past life. But their lives were returned to them as four young turtles and a rat in the present day, who were mutated into humanlike forms.
Splinter breaks out of his meditations to care for Donatello's broken body, which is still comatose in the lair. He thinks about how his son's soul is now inside a machine, and how he nearly died to save the entire world.
The other three Turtles are on a rooftop, watching the Purple Dragons prowling the streets. They think back on the destruction wrought by the Foot, including how Leonardo was brainwashed into subservience to Shredder. They also discuss how not all families are as cohesive as theirs, as evidenced by Casey's rotten relationship with his abusive father, Angel turning her back on the Purple Dragons, and Alopex's change of heart. Michelangelo teases Raphael about Alopex, and is told to shut up.
Then the conversation switches to their allies, and how the Mutanimals have helped them over time, along with Harold Lilja and April. Unfortunately, their enemies are still at large, and Shredder is now missing in action, and Donatello was near-mortally wounded by Bebop and Rocksteady. A still morose Raphael isn't entirely confident that Donatello is "alive" right now, but Michelangelo is confident that the Fugitoid will somehow restore their brother to normal.
The three brothers leap down from the rooftop to engage the street thugs, despite Raphael's surprise that Leonardo would break the rules. Suddenly Hun appears and tries to punch Leonardo, and deflects an attack from Raphael. Suddenly Bludgeon and Koya appear to bring Hun back to the Foot, but are intrigued by the idea of killing and eating the Turtles instead.
The three brothers start fighting their way out, only to be interrupted by a flamethrower. It's none other than Donatello (in Metalhead's body), and he drives back the two attacking mutants with a laser blast, as they vow revenge for Shredder.
On the way home, Donatello has a little trouble with the "car" mode he's able to transform into, because it moves faster than he is able to compensate for, and he crashes into a pile of trash cans. His brothers express gratitude that he's still with them, and decide to head home.
In a factory, Shredder watches with satisfaction as Baxter Stockman constructs hundreds of M.O.U.S.E.R.S.