This song finally creates a distinction between the play's characters and "the role of" each character, AKA their actors. In the song, it just says "Role of [Character]", but the novel names them Miku, Kaito, Meiko, Rin, Len, Luka, Meg, and Gack.
The song reveals (or clarifies) that this was originally all just a play, but the actors were sucked into the world to perform the play indefinitely. The novel goes into plenty of detail about the acting troupe; Miku is a novice actress who performs for the lead part of the play Crazy ∞ nighT.

At the beginning of the song, we see West End (presumably actually London's West End) and the theater. It's clear it's still a regular play in the first part - there's a visible audience, the set is just a stage, the "front door" is just there off to the side. Also, this is the only part of this song where simply "Villager" speaks, as Miku's in her role.
Miku becomes very caught up in the party that's part of act one (feeling similarly to how the Villager does, because she always dreamed of standing among these actors), and as she speaks her line about wishing for the moment to last forever, she makes a misstep and accidentally knocks the hands off the clock.
In the novel, this is followed by the others adlibbing to close out the act. They support her despite her mistake (in fact, they think it turned out really well), but she later finds a letter addressed to her on stage, then wakes up in the play without remembering what happened afterward.
In the play world, the others have become their roles, and it seems only Miku acknowledges the world's fakeness. She's horrified, and desperately wants to return everyone to reality. Of course, the hunt for the stolen page is going on all the while.

Now, let's backtrack a little. The letter Miku found on stage after act one revealed that the seven fabricated the "lost play" of Crazy ∞ nighT (presenting it as a lost script by the legendary playwright Burlet, who their troupe is dedicated to) to bring the troupe out of its financial woes.
Miku told the seven she intended to reveal their secret. The letter was seemingly meant to get them to have a discussion and set things right, but due to misunderstandings, Miku ended up running from the seven and falling down the stairs, the clock-hand knife (which she was going to show to Kaito to ask about fixing the clock) stabbing her in the chest.
When this happened, the seven wished that tragedy could have been merely part of a play. The Mastermind granted this to them via the endless play world of Crazy ∞ nighT: Miku's death was excised from reality and endlessly repeated in the world of the play, meaning she was fated to die in accidents during the course of it.
The seven can't let Miku know the truth, as it would destabilize the world built upon her still being alive. So they act like they've forgotten their real selves and are now part of the play, meanwhile trying to protect her from the accidents the world tries to kill her with. They also devise a plan that could, with many repetitions of the night, theoretically affect reality and prevent her death.
(So to explain the intro: It all began with Miku finding a letter on a stormy night, both parties thought they were betraying the other, her death was the "truth/page wanted gone" hidden in the coffins, and they keep performing the night to try and save her. The scream is Miku's accident, and the applause and buzzer mark the start of the play; Miku hears them when she picks up the letter.)

In the final loop, interested in what might happen, Len/Doll Boy brings Miku to the library, shown in the middle of the PV. There, she finds books describing the past loops. She's also realized (after Rin/Doll Girl tried and failed to steal it) that the letter in her pocket contains a blank page, the "End roLL" - which would usually translate to "credits," but it's considered the last page of the script, the place for the ending to go.
While Miku's mind forgets everything each loop, her body "remembers" the secret room in the basement and leads her there, so she learns about the coffins, but they're locked. She realizes that the End roLL, the clock hands, the script, and something to use as ink are all crucial.
When she goes to meet up with the seven and tell them about her discoveries, she overhears them talking out of their roles. Miku comes to think they've been repeatedly trying to kill her in "the proper way," such that her death could be "traded" with the death of their star actress - so she's just "the ninth person," the actress's replacement. Thus, she feels betrayed; she doesn't realize it's her they want to revive, and the other actress they brought up earlier is unrelated.
Once Miku has gathered the other essentials for the ending, she feigns ignorance to trick the seven and get the script, then runs off. (Incidentally, they go along with Miku's false assumption to keep her away from the truth.) After a chase mimicking the night of the accident, she eventually ends up in the coffin room, but wasn't able to keep the wine she intended to use to write with.
The minute hand unlocks the coffins (which contain the actors' original bodies and the missing page of Miku's death), and she finds her own dead body.

Miku remembers everything, including what the others did from her death up to starting the loop. (They read a message from the Mastermind at the start of the script which lays out their choices, then plan out how they're going to operate to save Miku. Also, it turns out the "page thief" was Luka: she tore out the page to make everyone forget the rest of the play and create an excuse to not have to follow the script. But while she did this independently, the others applauded her brilliance.)
Even knowing the truth, Miku still feels guilty about having killed the others on occasions such as Bad ∞ End ∞ Night. She feels she needs to destroy the play and force the others to go back to reality, even if she might not be saved there.
The part with her thinking about the others toward the end likely represents her remembering the actors from reality, and how she wants them to return safely after all their hardship in the play world.
She slices her wrist and uses her blood and the minute hand to write an ending ("The Everlasting Night has (is) fading away, here comes a new dawn") on the End roLL. Putting it in at the end of the script, her ending causes a fire that burns down the mansion, forcing everyone outside. As the play world is destroyed, they return to the "unwanted reality": the moment before Miku's accident.
While Miku ended the play before the seven decided they were ready, their plan succeeds after all. The final shot of a hand reaching for hers shows how, through their repetitions of the everlasting night, they were able to offset events ever so slightly that one of them successfully grabbed Miku's hand in time, rather than coming just short. The Mastermind applauds them for achieving the True enD.