Setup. Noctis and his friends are broke and have a broken down car
Part 1. Invasion and aftermath
Part 2. Gang meets up with Luna at Golden Quay looking information about the war. Luna has Shiva give Noctis her blessing but has to quickly return to the empire.
Part 3. The Gang now meets up with Ardyn who gives Noctis the Ring of Lucis. The gang take revenge on an imperial base (magic tutorial) before heading up the volcano to defeat the relocated Ifrit and recieve the first royal weapon.
This has now set the plot. Luna's family/kingdom are bad, while Luna has shown hints of going against them to help Noctis do what is right. Ardyn/Cor is a friend/Advisor who is at his most trustworthy after giving you the ring, a royal weapon and imbuing you with the responsibility of being king.
Act 2.
Part 1.Luna reunites with the gang now featuring Ardyn at Duscae and attacks them, trying to kill Ardyn. She claims that Ardyn has a darkness inside him, and begins winning easily. However after Noctis sides with Ardyn, Luna relents and lets Ardyn leave the party as Luna doesnt want to hurt Noctis. The Gang goes with Luna to recieve Titan's blessing since she can let them past security. Noctis recieves Titan's power, but things go bad, imperial troops show up and the gang are caputred by Ravus. Luna's Brother.
Luna's combat mechanics are just Aranea Highwind's copy pasted and named as Luna/Stella (or make Aranea's model the female lead)
Part 2. The Gang now find themselves in a secret imperial prison (The Game's Final Dungeon Copy pasted forward. They are rescued by Ardyn and escape with The Regalia after learning the empire's dark secrets, fighting Ravus and taking back Regis' sword. From here Ardyn guides the gang to
yet somewhat suspiciously stops at the entrance and leaves the gang to enters alone while he proceeds to scout out Leviathan in Altissia. On completion of the dungeon, Noctis recieves Bahamut's power and the ability to use the Armiger.
Part 3. Now with the use of Armiger there are 2 summons left to get. So the gang begin searching for Ramuh, adventuring around Duscae and searching for the mysterious final "Royal Arm" since now the game clearly guides you to all but one of the Royal Arms.
Summary. At first Luna seems just as complicit as her family and appears to set Noctis up. Luckily Ardyn saves them. Yet Ardyn keeps behaving in an increasingly suspicious manner and trying to pull Noctis down a more violent path.
Act 3.
Part 1: After exploring more of the open world, Sidequesting and finding most of the Royal Arms and having a meet cute between Prompto and Iris in lestallum.* The gang head to the lighthouse and their one way trip to Altissia. This is the point of no return.
After adventuring around Altissia, Noctis and Luna go together to meet Leviathan. Its very romantic, swirling music, the ocean backdrop ect. Yet there is a disconnect. Noctis seems different and Luna is significantly weaker then she has previously been. The Leviathan battle happens and Ardyn jumps in to help. Luna feels betrayed and attacks Ardyn but in her weakend state can no longer match him. Ardyn tries to finish Luna but is sent away by Noctis. Both Noctis and Luna are rescued seperately by their allies from this battle and there is a 2 week time jump for everyone to recover.
Part 2. The Race to the Capital.
Noctis and his friends
journey to the site of the final Royal Arm, Past Luna's Home and towards the capital. This plays out as a linear combat section that is Akin to a boss rush. (Or the final section of Bioware game Mass Effect 1 after the meeting with Vigil) The Guys are faced with seeing the illeffects of the growing night on the country side and develop an even sterner resolve to end the war with Luna's people and band together against the growing darkness.
Part 3.
The gang reach Insomnia and the cutscene/dream sequence of Noctis at the train station used in the Omen trailer plays out in real time. Under the cover of night (useful for hiding any draw distance technical limitations) the guys defeat another mech base and head towards the Throne Room. After slaying most of the Imperial forces a cutscene shows Noctis demanding a peace via surrender and have the agreement sealed by his marrige to Luna. Luna rejects this proposal, still harboring a distrust of Noctis after what happened at Altissia.
The player is given a choice. Throw the Ring of Lucis down in Anger/sorrow and walk away, or try and fight the imperial court including Luna and Ravus to get what they want through force. (Luna's fight mechanics are still Aranea Highwind's) if you choose the 2nd option Prompto switches sides and tells Noctis/the player that this is wrong.
It is at this point Ardyn re enters the picture. Explains his Malevolent goals and sets up the fight with Noctis. This happens in one of 2 ways depending on how you chose to deal with Luna's rejection.
1. Noctis accepts it and walks away.
Noctis, Prompto, Ignis and Gladio meet Ardyn outside the throne room. Ardyn reveals his intent. Noctis friends fight Ardyn and buy Noctis enough time to apologize to Luna, have a romantic moment and help the imperial court escape, before Luna fights off Ardyn as Noctis safely enters the Crystal
2. Noctis vs Luna
A battle ensues within the throne room that is complicated by Ardyn's entrance. Its a bloodbath. Everyone dies and Noctis barely makes it to the crystal.
Both Scenarios move forward the same way. Noctis enters the Crystal, powers up and exits 10 years later. Ardyn is waiting outside to finish the job. Noctis and Ardyn fight. Noctis wins, Yet both die.(just like Hamlet) Ardyn's death ends the plague of the stars and the world is restored, albiet in a damaged state.
In a manner true to Shakesphere's plays. FFXV ends as it began. The supporting characters who lived to see the end (Iris, Cidney, Talcot and a few other NPCs) are assembled in Hammerhead. These characters witness the first sunrise in a decade, and set out to rebuild the world.