It starts off with a flashback of Ryu sacrificing himself by sending himself and Evolto to the light as the others were in depression. What will happen next? Finale(?)
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It starts off with Akira standing on his two feet as the two worlds were colliding

Akira: "I'm going after him!" That was when Kazumi intercepted in his head

Kazumi: "Ryu was right to do what he done. He carries his DNA so he should be destroyed as well. Once the gap absorbs Evolto's energy the two world will combine into one."

Akira: "I'll add myself to the sacrifice."

Katsuragi: "Your ridiculous. But there is a chance where you can save the world." Then he regained his consciousness

Ravel: "Eh?! Akira are you crazy won't you die?!"

Akira: "Ravel, Ryu is my best friend. I can't just let him die." Then he was about to transform but Ravel grabs his hand and placed her Phoenix charge with the Phoenix bottle as she cries unbeknownst to him the bag had the cross-Z charge and the Genius Bottle by Sento

Ravel: "Akira... please come back alive got it?"

Akira: "Got it..." Then he placed it on his bag "Well Guys it was nice knowing you. And dad thank you." Then he inserts the hazard trigger and the FullFull bottle on rabbit form

Rabbit and Rabbit!

Gata Gata Gotton! Zuttan Zutan!

Are you ready?

Overflow! Kurenai no Speedy Jumper! RabbitRabbit! Yabei! Haei!

Akira: "Don't worry Ryu..."

Akira then jumped high to the light as he enters inside and find Ryu unconscious

Akira: "Ryu!" He went up to him only to find glowing red eyes aa it was Evolto taken over

Evolto: "Oh I'm sorry but unfortunately I taken over Ryu." Then he appears at his final form

Akira: "Evolto I'm here to end you once and for all!!!"

Evolto: "Speak with your action not with your words!!!" Then the two charge in and fight each other as Evolto has the advantage "What can you do your weaker than before!" Then Akira's armor disappear after another hit and then Evolto turns into Soichi just because "When will you learn that the word is better off without you or Kamen Riders!"

Akira: "SHUT UP!!!" Then he went to go and directly goes for a punch but Evolto blocks it and twist his arm

Evolto: "You are just a fake hero that was born from!" Then he kicks Akira away and reverts back to his final form "Face it no matter what you do you'll always fail as i'll rise to destroy everything. This is the end!" He was about to end Akira but something was stopping him

Ryu: "Hey Akira! What are you doing?!"

Akira: "Ryu?!"

Ryu: "I got this just go! Hey Akira you wanna know what type of face i'm making? It's me cracking a smile right now!"

Evolto: "SHUT UP THE BOTH OF YOU!!!" He then managed to shut down Ryu from speaking and then a bottle drop which was a dragon bottle but was silver

Evolto: "I've made sure he won't talk now."

Akira: "This is terrible..." He grabs the bottle as he use the fullfull bottle to go to rabbit form

Rabbit and Rabbit!

Gata Gata Gotton! Zuttan Zutan!

Are you ready?

Overflow! Kurenai no Speedy Jumper! RabbitRabbit! Yabei! Haei!

Then he runs towards Evolto as the parts of the rabbit forms him

Akira: "I promise to save Ryu!" Then he attacks Evolto "I may be a Kamen Rider but guess what I used this power to save people and for peace and love!" Then he gets flashback of him with Ryu "I'll be using this power to create a new world where i'll make sure nobody will ever suffer again. If I were to die I hope to be known for my deeds."

Evolto: "YOU'RE WRONG DESTRUCTION IS THE ONLY WAY!!!" Then he goes on the attack where he kept attacking to the point where he swapped to TankTank form "We will see who'll die first! Me or you!" Then he dealt another powerful blow where Akira downgraded

RabbitTank Sparkling! Yea- yeahhh!

Then Akira dealt a powerful blow that the things on his shoulder disappears as Akira used the HawkGatling to shoot Evolto but Evolto throw that weapon away and so Akira used the drill crusher and uses it but again Evolto throws it away where the claws disappears. Then Evolto dealt another powerful punch where Akira was at his first form RabbitTank

RabbitTank! Yeah!

Evolto: "And with that your back at the first form you took how pitiful."

Akira: "Evolto you forget that I have something to use." Then he pulls out the Genius Bottle from his bag and tries to use it but it didn't work "Wait how did this ge- oh wait...dad...it's not working...sorry dad." He puts it away in his bag

Evolto: "Guess it proves that I am strong." Then Akira's rabbit bottle changed color from red to gold

Then he summons his fullbottle buster as he grabs the rabbit and dragon bottle and insert them onto the buster

Rabbit! Dragon! Just match desu!

Evolto: "Do you think that would work?"

Then he closed it to it's blade form and then attacks but Evolto attacks him to stop Akira from striking but Akira jumps over him and strikes him over and over again multiple time

Just match break!

Akira: "You don't get it then because me and Ryu..." Then he strikes Evolto again "We are the best match!" Then he strikes him again causing Ryu to wake up

Ryu: "Hey dude you finally got me awake by smacking him!"

Evolto: "RYU... AKIRA YOU BASTARDS!"

Akira: "If my father defeated you then I would beat you as well!"

Akira: "Now... shall we begin the final experiment." Then he pulls out the rabbit and dragon bottles and shakes it as equations appear as he twist the caps and inserts them onto his driver

Rabbit! Dragon!

Are you ready!

Akira: "Henshin!"

Best match!

Akira & Ryu: "The formula for victory have been set!" Then he twist the crank

Ready go!

Then Evolto twist the crank as well

Ready go! Fever flow!

Vortex attack!

Then Akira was sliding down on a energy graph for his rider kick and as well for Evolto but Akira and Ryu were strong to push him back as the gold and silver reverted back to red and blue

Akira: "THIS IS THE END!!!"

Evolto: "I WILL NOT BE BEATEN BY SOME TEENAGERS!!!"

Akira: "It's too late Evolto!!! HAH!!!!" Then a explosion happen which Evolto was gone for good

Meanwhile back on earth A Vernage took possession for a small moment

Vernage: "Evolto is gone... my job here is done..." Then the golden bracelet came off Misora causing for Misora to gain control of her body and as the Sky wall were getting destroyed by equations and the tower destroyed as Akira was trying to grab Ryu and a shiny item. Then the two world fused into one as Akira wakes up holding a watch

Akira: "What's this?" Then he turns it to make a face and looks at the houses "There's no skywall."

Then Akira looks to see that the world was fine as he went around the city walking as then the news shows up with Namba still alive

Akira: "So the world is back to normal...then that means."

Katsuragi: "That's correct everything is back to normal."

Akira: "So I guess your dad was trying to put everything back to zero."

Katsuragi: "Zero? Now his research is no longer existed."

Then their came small moments. Gentoku working with the prime minister. Azazel started creating and researching sacred gears. Saji at school as he does what he was told by Sona. Kazumi at Nascita and to where he and the Hokuto trio were with him and then when he saw Misora he (idk if it's a nosebleed or nose smoke) and then the trio carried him. Ryuga was doing some practices for his next fight. Sento is on a date with someone. (A/N: "Guess who Sento is dating? 3) Ravel still in the underworld doing some reading. And Hygami was hanging out with Momo and Itsuki was actually confessing his love to Xenovia

Katsuragi: "We are in a world where you never existed. Though Sento Kiryu does but you didn't. Even though you survived nobody knows who you are. This is our last time seeing each other. Good bye Akira i'm sorry for everything I said and done." Then he disappears for good and then Akira notice Ryu and Akeno going out

A/N: "Confession to be fair I liked Akeno when she lets her hair down. I don't know why but this picture just... the only way I can describe is that she looks great."

Akira: "What the? Hey Ryu."

A.U. (Alternative universe) Ryu: "Huh? Do you know me?"

Akira: "Eh? You don't me?"

A.U. Akeno: "Sorry sir we have no idea who you are but do you need something?"

Akira: "No... sorry about bothering you." Then he walks away

A.U. Ryu: "What a strange person."

A.U. Akeno: "Agree." Then Akira went to Nascita though it was different

Akira: "Can I get coffee."

Misora: "Coming right up."

Soichi: "Hello there- eh wait you look familiar? Wait... You're Satou Tarou from Lynks! I'm a huge fan." Then he gives Akira his coffee "Can I get an autograph? Oh but have some coffee."

Akira: "Thanks." Then he took a hesitant sip and then it was good "Wow this is good

Timeskip

As Akira was sitting on the edge of a fountain he heard a familiar voice

???: "Hey Akira!" Akira looks to find it was the original Ryu

Akira: "Ryu?!

Ryu: "What's going on in this world. Nobody know who I am, there is some version of me who has black hair and was dating with Akeno!" He was angry at his A.U. version for dating Akeno

Akira: "I guess it's because in this world you don't have Evolto's DNA." Then he smiles

Ryu: "Man what's with that smile?" Then he started playing around with his hair

Akira: "This is terrific!" Then he press a button on his phone for it to change to his bike as the two ride the wind "Oh by the way your fly is down all the way."

Ryu: He looks down and see it was down "What?! How long have you noticed?!"

Akira: "The whole time before we fought Evolto."

Ryu: "And you didn't told me that?!"

Akira: "That would be the worst moment for me to tell you that idiot."

Ryu: "Who are you calling an idiot at least call me something else!" Then he shakes him causing Akira to swerve

Akira: "Hey stop!"

Later

The two were sitting on a bench as Akira shows Ryu a script

Ryu: "What's this?"

Akira: "This is a script of 63 chapters of our adventures including any fillers. Now time to look back on what happened." Then he press play on a little music machine which played a dramatic music "Kuoh city home to the country of Touto home to the genius Akira Kiryu aka Kamen Rider Build fighting the supernatural and the evil creatures known as smash the hero."

Ryu: "Hero? Genius? You're just some random person who knew about the supernatural."

Akira: "Shut it! And here is Ryuken aka Ryu Geki a somebody who is a dragon boy that out of my kindness I decided to make him Kamen Rider Cross-Z and along the way we made friends, enemies becoming friends, love, and everything around emotion..."

The end of the series

Let the experiment begin 2: Heroes reunited

Tune in with Akira Kiryu and Ryu Geki trying to reunite team build but however a new enemy approached and Akira lost his memories and as he not only tries to find out who he is but to regain back Team Build

Akira: "I'll hop my way towards for my memories and to bring back everyone I knew."

Akira: "Henshin!"

???: "Who are you?"

Akira: "I have no idea but when I find out who I am and everything about me I will be known as Kamen Rider... Springer. The one who'll hops his way to find his memories and to regain everyone I knew that were lost."

A/N: "And with that we are done with the series it's been only four months and I completed the first series. Thank you all for reading stay tune for "Let the experiment begin 2: Heroes reunited" on October 1, 2019