ok to sum it but to the best of my memory (haven't watched the movie in a while)
there's this kid who finds this giant robot (obv the iron giant)
he tries to work stuff out to give the giant a place to stay
government finds out, they want to destroy the giant
some stuff happens, something about missiles being launched at the city
they can't call the missiles off, iron giant goes to catch the missile and save the city
tells the boy (i guess they became like bros) "you stay, i go" (the kid told him that when they first met and the robot tried to follow him home)
kid's crying
robot takes off and as he's going he says "superman" (p sure the kid was telling him about superman at one point)
the giant hits the rocket and they both blow up
it's a really good movie, you should watch it
Actually, what happens is:
The Iron Giant saves two boys from death, but is spotted by the military and attacked. The robot tries to fly away with the boy he befriended, but is shot down by a missile. The boy becomes unconscious after the robot and himself are knocked out the the air, but the robot thinks that the boy has been killed. Enraged, the robot turns against the military and starts destroying everything that attacks him. The government man who had been searching for the robot earlier in the movie lies to the general who has a nuclear missile at his disposal. He says that the robot killed the boy and that it is now on a rampage throughout the city. Then the boy wakes up and shows himself to the robot, who stands down and becomes peaceful again. They both show themselves to the general, the government worker, and the rest of the military. When the general is told that the government worker lied to him, the general begins telling the nuclear missile launch to stand down, but the government worker snatches the walkie-talkie and tells them to fire the nuke. He then realizes that that means the missile will be headed straight for the city. He tries to flee, but is subdued and sent to prison. Meanwhile, the robot is told by the boy what has happened, and knows that the missile is heading straight towards the city which will kill everyone. The robot then flies off to intercept the missile. He takes it and carries it up towards space. Right before the nuke explodes, the robot remembers the story the boy told him of Superman. This is why he says "Superman". Then you see the explosion from the cities perspective, which means the Iron Giant has been destroyed.
Before all this, the robot was discovered by the boy who befriended him who tries to keep him hidden from the government. The robot is almost like a mentally disabled little brother to the boy, because the robot doesn't know much about Earth and doesn't know what's right and wrong. This is what makes the robot's death so sad.
It also helps if you originally watched the movie when you were a kid.
~90s kids will get it~