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Tear Jerker: Astro Boy
Tenma finally giving up on his schemes to reclaim Astro as his son: "I'm sorry... I give up... I just wanted Astro back, that's all... I thought if I could get a brand-new Astro, he could be my son again... so I tried to use Meeva to bring me a new Astro from the past... I know it was a crazy thing to do... but when I learned I couldn't have a brand-new Astro, I thought maybe I could get you to let me have the old one... I'm sorry... I know it's not possible... you win... I know you're his real mom and dad now... you're both wonderful parents... I know you can make him happy... I'll never come here again... I promise... give my regards to Astro..."
Dr. Tenma/Boyton's reaction to his son's death... the worst being when we cut to him in his office, sobbing into his desk...
And then later on in the episode is his rejection of Astro. Goes from saying he'll teach his son how to fly and jump mountains to ordering him angrily to get out.
From Snow Lion: "This is how it ends: in the snow. If it's the last thing I do, I'll put Astro Boy in the Robot Hall of Fame. Yes, Astro Boy, you've earned your place here with the best and here you'll stay for eternity... and here I'll wait for you, my friend. We'll face the end together."
Tenma's desperate search for Astro in the second episode. Curse you, Hamegg!
Tenma:(calling for him) Tobio, I'm sorry. You are my child.
Astro:(trapped in a suitcase) Father... I'm here... Help me...(slips into unconsciousness)
Nuka literally falling apart in Astro's hands.
Atlas' death five episodes into the series, though he later is rebuilt.
Pluto's Heroic Sacrifice where he drags a mindless clone of himself into a volcano to prevent him from harming Astro and Zoran.
Astro's heartfelt speech to the Blue Knight about wanting humans and robots to live in harmony, with just about every character in the show watching via television.
The hug between Astro and Tenma in the final episode.
To anyone that read the original or knows anything about Astro's creation will see this coming as soon as Toby appears. Toby dies. Toby get resurrected by Tenma into Astro. Astro doesn't act like Toby that much. Tenma disowns him. It's the thing you expect the writers to have just glossed over... gets fully shown...
Granted, they completely changed Astro's cause of death (he gets hit by a car in both the manga and some of the adaptations), but it otherwise might have been too scary for kids.
Tenma's collapse after Astro's heroic sacrifice. Just when Tenma realized how much he really did love his robot son....
When Tenma disowns Toby. This is mostly because of a truly harsh line delivered in a rather unique fashion. "And I dont... want you anymore." Somehow, Tenma manages to both sound just a bit confusingly relieved... and absolutely horrified that he's thinking this way. Even to those who know it's coming ,the line is INSANELY cruel.