My theory based on interviews and other ideas I've read
Retribution kicks off with Afterlife's payoff with a battle zone aboard the Arcadia. While everyone scrambles among explosions and the invading Umbrella soldiers, Alice loses sight of her friends Claire Redfield, Chris Redfield, and KMart.
She becomes distracted briefly at the sight of what appears to be her old friend Jill Valentine, who seems to be in charge of the invasion. Near shore, Luther recognizes his friends in trouble and quickly assists them by using a nearby tank, taking down a couple of aircraft himself. The odds are overwhelming though, Alice barely evades Valentine's gunfire and jumps off the Arcadia as a last attempt to save her life. As Jill approaches Alice, she receives orders from Wesker via tech device, to capture Alice and her friends instead. Jill is displeased, but like Elle Driver in Kill Bill, ultimately obeys.
Alice awakes in the clandestine facility where she is heavily interrogated by Umbrella officials. Why did you turn on Umbrella? What is your agenda? Alice is silent for days on end. FLASHBACKS before the outbreaks.Alice also questions silently, why Jill has turned on her.
Meanwhile Luther meets Leon Scott Kennedy along Barry Burton and other survivors. A convoy similiar to Claire's from Extinction. They search for Alice. Leon and Barry are privy to Umbrella's secret location and entrance where they're certain Alice and co have been taken to.
Wesker's other assistant Ada Wong shows up to aid Alice in escaping her cell. Alice does not trust her, but she has no choice.During their adventure across the globe...they encounter the undead and armies of Umbrella soldiers, among them, clones of old friends including Rain, Carlos Olivera, and One. Ada informs her that there are many more. The soldiers and army of clones are led by an irate Jill Valentine hot on their heels.
As they head on, they encounter a group of Licker eggs. Alice recognizes a clone, that looks just like her daughter Rebbecca (Chambers nod, maybe??) from her old life before the outbreak..and before something even more sinister. Although Ada confirms her as a clone, Alice brings Rebbecca with them, much to Ada's disagreement. As they near a secret exit, The women are ambushed by Jill Valentine and her troopers. One of them, a Rain clone who is closing in on Alice. Somehow, Alice talks some sense into her, just as a group of undead is unleashed. Ada distracts Jill telling Alice to run, assuring her that she will go another route turning the heat on herself. Meanwhile Alice and Rebbecca are overcome by the undead, Alice using guns and chains to try and survive, Rain jumps into the fray, aiding Alice in her escape..out of the facility..that is set to explode at any moment.
Alice and Becky(short for Rebecca) emerge somewhere in Russia, where Leon, Luther, and Barry(and co) approach them in a hot Rolls Royce. At just a brief pause of relief, a giant, mutated Licker attacks, causing the good guys and gals to speed off.Not only is the Licker hot on their tail, so are the mutated plaga infested undead. They're riding motorcycles too and intent on killing Alice and co. Leon, Luther and Barry take out the ganados, and Alice takes out the Licker with her mean driving skills.
The group is invaded again however, this time by more undead and Jill Valentine and her troopers. Jill and Alice go at it while Leon and the other men cover Rebecca and take care of some business. Eventually they are able to escape, just barely...one of their group, is severely injured.
Based on some reliable information, Alice's friends, Claire, Chris, and KMart are possibly in another Umbrella location, a base underground the Antarctic. Alice leads the group, towards the base as more FLASHBACKS take place. Some characters have known each other previously. Leon and Jill for example. Leon also stumbled upon Plagas somewhere in Europe, hence his skills on dealing with them during the previous battles.
As they arrive, Jill Valentine, Bad Rain, more soldiers and a captive Ada Wong crash the scene before Alice is able to investigate further. Leon, Luther and Barry are ready for another battle as Rain takes down Ada, who is quickly saved by Leon...Alice and Jill Valentine have a war of words, where it is revealed, her friends are not in this underground base and more importantly, that ALICE was born and created, to be part of a superior species of human beings. This is why her DNA bonded with the T Virus years ago. Not only has she been brainwashed, monitored and exploited for years, she serves a more blood tied purpose. She is the sister of Albert Wesker whom is also created as she is, and are part of a group of Wesker Children. Created, designed, and guided, by their father, the founder of The Umbrella Corporation, Ozwell Spencer, who despite his old age, is still alive somewhere. Monitoring, still tracking Alice, who sent for his daughter, to finally come home.
Alice is in shock, but after a moment, realizes, her family are with her(Becky, Leon, etc) right now. Her family are also Claire, Chris and KMart, whom she won't quit looking for....Jill asks her to decide. Come home Alice, or death is the other choice.
Jill looks towards Rain who has boosted her strength via a Plaga serum, "Is that all you've got?" Alice asks looking at Jill.
"It's more than enough...Alice..."
The epic battle ensues. Blood is spilled, and midway through the battle, Jill's battle suit becomes shredded, partially exposing the red scarab on her chest. The fight continues, both Alice and Jill Valentine injured heavily, but finally...Alice is able to take out the device and killing Rain clone again in the process. Jill is knocked out as Alice looks towards her friends and back down at Jill. She retrieves the communication device attached to Jill's waist and before she can figure it out, Wesker and Ozwell Spencer appear on the tiny monitor. Wesker informs Alice that her friends are still alive, but not for much longer as through the device, Ozwell's eyes pierce into Alice's. She stares back into his icy, yet inviting eyes. Alice in disbelief.
End Movie
Post Credits
(SECRET ENDING)
Claire Redfield, Chris Redfield and Kmart are shown escaping their own cells..when they are approached by Wesker and some Umbrella soldiers, their fate is left unknown.
((Sorry for the long post, but based on interviews and bits of data given in interviews etc, I kind of figured this might be close to the movie.))