and Peeta is gone, what happened to him at that point? Was the original plan all along to use the tree to kill the remaining Careers? And, what was the original plan on getting her out, because she obviously did not know what was going on.
God loves you as if you were the only person on this earth.
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It's not explained why Peeta was not at the tree, only that they could not extract
him. The plan was to use the lightning tree to kill the careers on the beach. I think
the plan was to keep Katniss alive until a situation arose when they could extract her.
No, the plan wasn't really to electrocute the Careers on the beach. That was a cover up so they could tie the wire to the tree without suspicion. Beetee's real plan (which Finnick and Johanna knew about, but not Katniss or Peeta), was to tie the other end of the wire to a knife, and then, when the lightning storm hit, he would throw the knife into the force field and shut the arena down, making it possible for Katniss to be extracted.
Unfortunately, it went wrong, Beetee ended up electrocuting himself (you'd think he'd know how to avoid that, but apparently not). Katniss found him, and when she saw the knife and wire, she relized what he had been trying to do. So she attached the wire to one of her arrows instead, and shot it into the force field that way.
She did not, however, know the full plan at that point, she just thought she was making an act of rebellion against the Games ("Remember who the real enemy is").
As for Peeta, we don't know exactly where he was during this, but he got split up from Finnick somehow. In the book, he answers some of Katniss's calls. We later find out he was fighting the Careers (maybe with Johanna, who was leading them away from Katniss?), that's probably why he couldn't join her back at the tree. He killed Brutus, and maybe he, Johanna, and Enobaria were then picked up from the same spot by the Capitol? The movies don't even tell us what became of Brutus.
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"The best fairytale is one where you believe the people" -Irvin Kershner