Here's a summary of episode 1:
It starts off with Riko and Nat (the boy that was with her) cave raiding in the first layer looking for relics. A little while after than Riko finds Nat unconscious and bleeding with a "Crimson Splitjaw" thats usually only found in the 3rd layer that was about to eat him (remember than name for later episodes). Riko gets its attention by using the whistle and it chases her towards the edge of a cliff and just as it seems that she's done for she's saved by an energy blast that hit the monster from quite a distance away.
Riko follows the trail of destruction that was left by the energy beam which lead her to the "Robot" aka Reg as we know him now. Riko and Nat take him back to the orphanage and sneak him in there without being noticed. Later that night they wake up Reg using electricity which takes out the power to the rest of the orphanage and "Leader" is coming towards Rikos room as he suspects its most likely her doing. Reg who has lost all his memories of himself learned again that he could extend his arms like a grappling hook which helped him avoid being found out. Later on just as it's about to be daybreak Reg and Riko escape from "Leader" without being noticed by using Regs extending arms to get out of the orphanage through the window. They get to a tall hill to where you can see the abyss and the entire city that surrounds it as that was what Riko wanted to show Reg and she explains to him that he mostly likely comes from the deepest layers of the abyss.
And then before the episode ends the Narrator goes on to say:
"About 1,900 years ago, a huge pit was discovered on a remote island in the southern sea of Beoluska.
With a diameter of around 1,000 meters and a depth that is still unknown to this day, the mysterious formation mesmerized people.
Valuable and dangerous primeval creatures and mysterious Relics that are beyond comprehension beckoned adventures looking to strike it rich, which in time gave rise to a giant city.
Over the span of many years, with a spirit of adventure for the unknown and countless legends luring them in, the worlds remaining unexplored chasm has swallowed up a great many people.
It is known as the Abyss."