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Nearly killed by nuclear war, the sentient spirit of planet earth , know to many as MOTHER EARTH, floods the planet, culling the human population. Scattered across the livable land in independent territories, the humans have one last chance to prove their worth to the planet . Most technology has been replaced with magic, but a handful of people cling to science, surviving with solar technology and genetic modification.
17-year-old SEREN MCINTYRE is heir of ASSANA MCINTYRE, the ruler of a sovereign territory called Valley-Port and an ambassador between Mother Earth and humanity. Their connection to the planet allows them to manipulate the elements. However, that connection works two ways. Every time a mage like Seren uses magic, they risk being controlled by an elemental spirit or Mother Earth Herself. 
Growing differences between territories create the potential for a war, something that could lead Mother Earth to rid Herself of the remaining humans. Therefore, dozens of small territories, including Valley-Port, decide to band together as one nation at the inaugural Newly Unified North Eastern State’s (NUNES) convention.
The convention is being hosted 60 miles north at Merry Basin by TREE-STRONG, a mage whose power rivals Assana’s. Since local elemental spirits won’t allow Assana to leave Valley-Port, Seren embarks to represent Valley-accompanied by peacekeeper twins, DAVID and REGGIE. After a day of travels, they arrive in Little Port where Seren meets up with their boyfriend, ERIK. He warns them about Altzis, a group of bigoted strangers loitering in Little Port on their way to NUNES. 
A storm blows up when Seren leaves Little Port, but the ocean spirit is one of the few elementals Seren trusts, so they draw power from her to get through the storm. They make it to Port’s Mouth just before nightfall but are run aground by a sola-powered barge piloted by scientist who fear mages. Seren pulls their boat out of the water to repair the damage, but can’t find a part they need. Seren makes a bargain with a local smith: retrieve a machine trapped in the GREAT SALT MARSH and he’ll make the missing part.
When Seren returns to Port’s Mouth with the machine, they’re attacked by Altzis. David and Reggie try to fight them off, but are outnumbered. A desperate Seren overcomes their fear of the elementals to draw power from an unknown river spirit, PISCATAQUA , who takes over Seren’s body. Piscataqua uses Seren to scare the fighting humans with a tsunami, making The Altzis drop their weapons and flee.
After a brief stop in a hostile town, Seren and the twins camp on a riverbank. Local elementals snatch Seren’s spirit from their body and put it in a bird circling an Altzi ship. They discover Erik is held captive on board while the Altzis interrogate him about the elementals defending Valley-Port.
The elementals just want Seren to sink the ship and kill everyone on it. Seren manages to hold the Altzis and raging elementals off long enough for David and Reggie to sneak on board and rescue Erik.
The group makes it to Merry Basin without further incidents, but their troubles are far from over. The delegates, a mix of mages like Seren, Altzis, scientist, and village leaders who lack magic, cannot agree on anything. And Tree-Strong turns out to be so bad at moderating that Seren fears he is trying to cause a war.
With the help of impish elementals Seren snoops around Tree-Strong’s home, but every time they get close to his office, his security systems nearly kill Seren.
The elementals and Mother Earth grow weary of the human squabbling. When disagreements lead to violence, She uses Seren to give the humans an ultimatum: come to a peaceful agreement by dawn or die. A storm blows up. The more people fight, the stronger it gets. It becomes lethal when a man tries to blow up the pier where most NUNES delegates have docked their boats.
The man appears to be an Altzi, but Seren discovers Tree-Strong used magic to force him to deliver the explosive.
Seren finally bypasses Tree-Strong’s security and finding evidence that he engineered Altzi movement in hopes that by scarring people and saving them, they would accept him as their ruler, making NUNES an empire instead of a republic.
Seren imprisons him and moderates the final discussion. The delegates vote to approve the constitution.
As the sun rises, waters recede, and a rainbow stretches across the land -- a sign that The Mother approves of the outcome. Seren returns home no longer afraid of their power and more confident in their ability to one day fill Assana’s shoes as an Ambassador between Earth and humanity.
: I’m not positive, but I think this should be capitalized as Planet Earth
: Better clarity if you say they’re trying to prove themselves to Mother Earth
: Does ‘they’ refer to Seren and Assana? Is Assana a man or woman? Reading through the synopsis, I think Seren is agender, but since the ‘they/them’ pronouns might be unfamiliar to some readers, it’s worth pointing this out upfront
: Also, what do you mean by their connection to the planet? Can you add a little more detail here?
: I would use ‘mages’ here, so it aligns with the plural ‘they’ later in the sentence
: What does it mean for people to be controlled? I assume that’s a negative thing, but why?
: I’m wondering why Mother Earth bothered to keep any humans around when she flooded the planet. If she’s just waiting for them to start a war so she can render them extinct, why not just get rid of all of them when she had a chance?
: You’ve got a lot of character and place names in these first few paragraphs. Unless the place names are significant, I’d omit them and refer to places more generally.
: Should be Valley-Port, right?
: Whose boyfriend is he? The twins’? Seren’s? I’m confused who ‘they’ refers to here.
: There are way too many character names now. You should stick to 4-5 named characters in a synopsis. Skipping ahead, I think you need Seren, Assana, Mother Earth, Erik, and Tree-Strong. The twins can be referred to as ‘the twins,’ and anyone else can be referred to generically, too.
: This should be ‘solar-powered’?
: This doesn’t need to be named
: Here, I’m not sure if the ‘they’ refers to Seren or Seren, the twins, and Erik.
: Don’t need this proper name. Just say ‘river spirit.’
: Nice stakes.
You’ve got a clear plot through-line here, and I have a good idea of where the story goes. Most of my comments are for adding clarity. Doing that through deletion of excess character names will get you a long way. Best of luck with this!