Keep in mind that most of the information here is taken from the fanwiki. 

This post is focused on Round 1 ! ( spoilers free )

Topics: 

1. What is ALNST even about ?

2. Round 1 Characters !

3. Where do I watch it? In what order?

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1. What is ALNST even about ?

Alien Stage, a reality audition program, has taken the alien world by storm, and its upcoming season is planned to be just as exciting. Contestants must sing for their survival, as the loser faces on-the-spot termination. As the competition heats up, a mysterious girl emerges, whose involvement may hold the key to humanity's fate...

Alien Stage is a youtube series produced by Vivinos. Every "round" ( episode ) has it's own song. In this series alien have taken over the world and make humans sing for them in competitions, whoever loses dies.

Round 1 is the round where Mizi goes against Sua !

2. Characters 

Mizi

Mizi is a young adult with fair skin and long pink hair that gradually transitions into a darker blue hue. Her eyes are golden in color, and she wears glasses when she is outside of the competition.

In the first round, Mizi wore a short black dress with an halter bust and a belt accentuating her waist. The look was paired with thigh high boots in a similar shade and mismatched gloves, one of which was shorter than the other. A blue gem can be seen sitting between her collarbones.

In a now-deleted tweet, VIVINOS mentioned that Mizi was the most outgoing participant and always carried a positive attitude. However, it has been noted that she finds it difficult to hide her true emotions and struggles with being dishonest. 

Additionally, it is implied that Mizi was raised in a favorable environment, which may have contributed to her sensitivity towards the darker aspects of the world around her.

Sua

Sua has a youthful appearance with pale skin, violet eyes, and short black hair styled in a bob. Her idol outfit consists of a short white dress with a large bow sitting on top of the hem, and long puffy sleeves that cover her arms completely. She accessorizes with a crescent-shaped headband, pearl earrings, and white ankle boots.

In a now deleted tweet about her, Vivinos describes Sua as one of the more elusive participants, as she rarely interacts with anyone other than Mizi. She has a tendency to surprise others with her bold and unexpected behavior and is the type to not be bothered by other people's attention.

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3. Where do I watch it? In what order?

You can watch Round 1 in Vivinos channel on YouTube 

Keep in mind that the video contains blood though. .

You can also watch “Sweet Dreams” in the same channel which is like a “trailer” for the series 

Basically:

Sweet dreams > Round 1 > Round 2 > Round 3 > Round 5 > Round 6 

Have fun watching! Keep in mind that Round 4 does not exist because it was an irrelevant round . . ( Main cast character VS secondary character )