CW script/plot supposedly leaked at 4chan (spoiler it if discussing for those who doesn't want to know, maybe label CW so others do not mistake it for AoU spoiler? Thanks!):
The movie is set one year after Ultron's attack. SHIELD leaks have raised awareness of the circumstances of several incidents such as the Battle of New York, the Mandarin bombings, the Dark Elf invasion, which, in addition to the Hulk rampage in South Africa and Ultron leveling Sokovia, have caused superheroes to experience a sizeable public backlash. Stark is retired from being Iron Man and dedicating himself to philantrophy to atone for his mistakes.
Captain America is leading the New Avengers - Black Widow, Falcon, War Machine, Scarlet Witch and Vision - to take down the remnants of HYDRA and find Bucky, aided by Nick Fury.
The movie opens with them busting a drug deal and recovering some Vibranium-based weaponry. Back to their new headquarters, the Avengers get to mingle. Black Widow is the co-leader, Falcon and Captain America are true bros, and Falcon also has a sort of friendly rivalry with Rhodes since they're both airmen. Cap is coaching Wanda into dealing with her powers and losing Pietro, and Vision is dealing with his expanding consciousness.
Fury drops by to share some info on Bucky. He's been killing HYDRA officials all over the globe, and it's getting hard to keep track of him or cover up his assassinations. Lots of the HYDRA assets he's offed are power players with seemingly spotless records, and the public wouldn't understand.
We then see Spider-Man's first public appearance catching some criminals, which switches to a Congress hearing discussing the matter, with some politicians advocating for the Superhero Registration Act. Stark is in attendance alongside T'Challa, who is acting as a spokesman for Wakanda on the matter of the Vibranium stashes that have been exposed by Ultron's attack. The UN believes Vibranium to be a dangerous resource and wants to oversee its mining and safekeeping, but T'Challa defends it belongs to his people. The usual political intrigue.
Cap and Stark meet to discuss the Vibranium weapon usage and discuss the concept of Registration, with Stark supporting it and Cap being against it. They discuss their motives. Meanwhile, in HYDRA's main lair, Zemo kills people who have failed him. Crossbones is there. Zemo says they in fact didn't fail, but it needed to appear so, because he doesn't trust anyone. And it's time to put their plan in motion.
Acting on a tip from one of his contacts, Bucky arrives at a HYDRA outpost in London, where he is ambushed by mercenaries. It's a set-up. The fight spills into the street, where Crossbones arranges a bombing to make it appear as if Bucky caused it. The explosions kills several people and causes widespread damage. Bucky escapes, and the Avengers move in to clean up the damage. The public outcry is massive, and the SHRA is approved. Stark decides to support it. He has learned from going through SHIELD leaks that Bucky killed his father and wants to bring him in.
In his farm, Hawkeye is attacked and captured by Crossbones, who kills his wife. He's then taken to HYDRA's lair and tortured for infomation in Fury's secret network, which only Barton is privy to, but doesn't break.
Cap is called to Stark's office, where he meets CIA asset Peter Gyrich, from London, who has been partnered with Sharon Carter. They announce the Registration has been approved and Cap and his team need to surrender. Cap says "no". Stark tells Steve he's bringing Bucky down, Cap says Bucky is innocent, brainwashed and needs help. Gyrich orders soldiers to seize up Cap, but he escapes.
Cap returns to HQ and tells the others what happened. He gets into an argument with Black Widow, who believes they need to register. They exchange harsh words, and when the military come to collect them, Cap leaves with Sam, Wanda and Vision while Natasha and Rhodes stay behind and join the Registration.
Fury sets up the Avengers in a safe house while Stark approaches Spider-Man to join the Registration. Spider-Man confides with Stark his secret identity, and Stark is surprised to learn he's just a boy. They bond, and Peter mentions Uncle Ben and his mantra that "With great powers, come great responsabilities". Stark then unveils his team on public TV, while Cap and his team become fugitives. Their top priority is to find Bucky before Stark.
Fury gets a lead on Bucky's whereabouts and the team heads there, but it is an ambush by Stark and his forces, leading to a fight. Stark has a new Bleeding Edge armor, but Vision holds his own. Scarlet Witch BTFOs some soldiers and Spider-Man, Falcon VS. War Machine, and Cap VS. Widow while she tries to get him to see things from her point of view. Scarlet Witch manages to blow them back long enough for her team to escape and reunite with Fury, who deduces Romanoff used her intel to forge a Bucky sighting to get them into a vulnerable place.
Cap doesn't want to endanger anyone else, but they all decide to stick to their guns and help him. Stark learns from Rhodes about the Vibranium weapons they busted from HYDRA, and Gyrich reports it to his superiors. They accuse Wakanda of endangering global security by not giving them entrance, and tensions escalate.
Bucky finds out about the Vibranium crisis and goes through his old contacts, eventually locating a HYDRA outpost where they storage Vibranium stolen from Wakanda before shipping it off to the other bases to produce weapons. Stark locates Bucky and tries to bring him in, leading to a fight in which Bucky gets pretty roughed up, but escapes. With no other option, he makes his way to the safe house where Steve is located, which he's aware of due to his extensive research on the SHIELD leaks, and tells him everything.
The New Avengers patch Bucky up and he and Steve finally talk it out. We get some exposition on Zemo and the reaffirmation of the Steve/Bucky broship. Meanwhile, Stark reports to his superiors about the HYDRA stealing Vibranium from Wakanda to produce weapons. T'Challa assures that they'll handle it on their own, but the government is undeterred and Gyrich orders the government Avengers to attack Wakanda, which Stark reluctantly agrees to do. Sharon, who is often at odds with Gyrich over his M.O, contacts Fury through a secure line and relays the info to Steve, saying she has faith in him.
Meanwhile, Hawkeye escapes from Zemo's hold as we've established.
Stark's Avengers arrive on Wakanda to seize the Vibranium, leading to tensions with the Wakanda military despite their efforts to maintain peace. Steve's Avengers intervene to stop them from causing an international incident and instead shut down the HYDRA outpost collecting Vibranium. Another fight ensues, in which Maria Hill is outed as a mole who sabotages the Bleeding Edge armor, leveling the game for Steve and Stark to have a big fight. The other Avengers fight as well, and it spills into the street.
Steve notices that, while everyone is beating eachother, Spider-Man has peeled off and is rescuing civilians. He realizes they're fighting for themselves, not for the people, and they need to work together if they want to stop HYDRA. Natasha, who had been having some disagreements with Stark, vouches for Cap as they realize they're stronger together than apart.
T'Challa intervenes and volunteers to assist them in stopping HYDRA because he wants peace between his father's country and the United Nations. The Avengers, however, do not find the HYDRA outpost. It was all a dupe so they'd kill each other for Zemo.
Hawkeye manages to contact Fury and report to the location of Zemo's lair: Switzerland. The Avengers head there and rendzevous with Barton as they mount a massive attack on Zemo's compound. Zemo orders his men to fight back using Vibranium-based, reverse-engineered weapons from the Chitauri and Ultron, and the battle is fierce.
Hawkeye fights Crossbones and kills him with four arrows, one for each of his dead family members. Black Panther kicks ***. Wanda eventually manages to achieve enough control over her powers she rearranges the molecules of the HYDRA mooks weapons, culling the Vibranium's special properties and allowing the Avengers to solo them.
Cap and Bucky tag-team Zemo and his Vibranium sword, but Zemo still kicks a lot of *** while rambling about his ideals. He eventually gets defeated and Bucky moves in for the kill, but decides to spare him, and prove that he is more than the assassin HYDRA forced him to become.
Zemo and his men are locked up, and Stark retrieves the location of all of HYDRA's secret bases. The military move in to dismantle the entire organization, arrest its figureheads and aprehend their weapons and technology.
The Avengers are still wanted, but Stark decides to let them go, and Fury and Jason produce new identities for them to lay low. Vision decides to leave the Earth. Cap decides to surrender, but entrusts his shield to Bucky, so he can always remember who he really is: a hero.
Cap is arrested. Barton finds out his children have survived and leaves with them and brings Wanda along to look out for her as well. Natasha and Rhodes are back with Stark, while Fury disappears once again, now with Hill by his side. Peter realizes he isn't made to fight in wars, he's there to look out for the common man, and Stark sends him off, erasing his data from the registry.
Stark BTFOs Gyrich, announces he's taking over because Gyrich made illegal calls throughout his involvement with the SHRA, and Carter is promoted to his place. Stark visits Cap and they patch things up, Zemo is locked up in the Raft, Natasha visits Cap and they patch things up, Bucky keeps the torch alive.