This masterpiece is my favourite music video ever. The song and the choreography are great by themselves, but the video was just flawlessly executed and contains copious symbolic devices.
1) The original Paradise Lost is about the fall of mankind due to Hawwah eating the forbidden apple, listening to the advice of the serpent. The forbidden fruit is biblical symbolism for sex, given that creation of life is within God's domain, and yet it's so tempting because sex gives pleasure. The original story is written in a way that puts the blame on Hawwah, drawing Adam as a pious innocent servant of God, a victim of the sinful nature of women. Gain reinterprets this as Hawwah making her own proactive choice of shunning the paradise devoid of free will.
2) She starts in paradise, which is the place with the church organ, which I think was deliberately illustrated as not very appealing. She listens to the serpent's advice and hesitantly moves to the tempting forbidden fruit. The fact that the fruit is sex is illustrated by that she rides the 'bed' (lift) to quench her thirst from the pipe in a provocative manner, past the door on which there's the Tree of Eden symbol.
3) Hawwah looking into the mirror shows her inner contemplation and the process of developing free will. The clock in the mirror represents the price of disobeying God's worse-than-China zero-child policy (corollary of no-sex policy), which is mortality. Admiring her beautiful self in the mirror, she chooses with her free will to identify herself as a seductress. The light blinking symbolises her gradual and hesitant moral transformation, and when it goes off completely, she makes up her mind and shuns paradise.
4) Gain in Black and White: At first, the black is the snake, the voice of temptation. Gain in white is Hawwah's pure and innocent state before she commits the sin. As she commits the sin, they overlap and mix, showing how from pure innocent one dimensional domesticated animal of God, she evolves a complex character with both good and evil.
5) The serpent and Hawwah are equated with the snake-choreography as mentioned before, which means the sin is internalised. But the snake and Hawwah were one from the start, shown by Gain's long white dress which resembles a snake in the beginning. Snake is just her internal voice of temptation materialised.
6) The ending scene is the best bit: amongst the damned men kneeling and bowing in pain and shame, she is alone in the light. Although she's damned just like them, she faces God, maybe fearfully, but without remorse nor regret, even putting a dim smile onto her lips at the end. She lost Eden, but got another Paradise, as in the lyrics.
7) I see feminist messages on many of Gain's songs. Bloom talks about female masturbation, Fxxk you is basically 'no means no.' PL is sexualised of course, since the topic is sex. But it is not mindlessly so and it "subjectifies" rather than objectifies women, unlike the majority of other K-Pop songs. This brings into discussion the topic of women assuming roles as active decision-makers in sexual relationships, because they were the decision makers from the very beginning. Note how there isn't a single guy in this music video until the very end, when we see damned creatures devoid of character in contrast with Hawwah, a remorseless rebel. If she wanted to be just sexual, she could've just done it like Hyun-A (I'm also a huge fan of her, but none of her songs or music videos show this level of artistry).