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General Heckler first appeared accompanying Stone as the former speaks with Tenma about the Peacekeeper demonstration and watched Elefun's presentation about the Blue and Red Cores. He later witnessed the Peacekeeper using the Red Core, shocked of its potential for destruction as a projectile was fired towards him and had to dodge it. The death of Tenma's son Toby further cements his view.
As the ship they were in was cruising through Metro City, Heckler was the first to recognize a robot detected with the Blue Core as Toby Tenma (renamed as Astro Boy). He commanded the units to capture Astro as they chased him throughout the city before Stone ordered to fire at the boy, launching Astro off Metro City and into the Surface. The general soon headed over to Tenma's and placed Elefun under arrest and in handcuffs as Stone interrogates both scientists about the Blue Core's whereabouts. After Tenma made an agreement to shut down Astro Boy and hand over the Core, Heckler released Elefun and leaves with Stone and the troops.
Sometime later on, Heckler and Stone were able to locate Astro Boy in a fighting arena owned by Hamegg. By that time, the two arm wrestled, with Stone winning it. Things were turning out alright as they gave the robot to Tenma for him to remove the Blue Core; however, the scientist, driven by the memories of his son, decides to help Astro escape with the Core. Heckler soon became horrified when Stone recklessly placed the Red Core in the Peacekeeper and saw him being absorbed by the robot, and evacuated with his soldiers as the Peacekeeper, now merged with Stone, started to absorb every surrounding objects.
He was not present during Stone's fight against Astro Boy until the aftermath where Heckler is seen being carried away by guards along with Stone, being arrested for his crimes. It is heavily implied that he was sent to prison along with Stone.
We're trackin' an unidentified object flyin' over Metro City, sir.
~ General Heckler alerting Stone.
Is that...Tenma's boy?
~ Heckler upon seeing Astro Boy as the scientist's son.
General Heckler being arrested and carried away by the soldiers along with President Stone.
Despite being by Stone's side for most of the movie, he hasn't done much contribution aside from when chasing down Astro Boy.
His entire character was likely created after Stone was written to be the president of Metro City, as the latter was originally supposed to be general instead of Heckler.