Most of the games in the Baldurās gate Series are pretty old now, hard to play even remastered. You can look recap videos if you want to know the story.
As a a new player in the series, the best is probably to start with Divinity Original Sin 2.
Divinity Original Sin 2 is in promotion today.
$30 (-38%) on Instant Gaming
Instant Gaming does massives buys of Steam keys, thatās why its cheaper.
I buy nearly all my games with Instant Gaming since I discovered it.
https://www.instant-gaming.com/fr/997-acheter-gog-com-divinity-original-sin-ii-pc-jeu-gogcom/
I spent 100+ hours on it to finish the story once, so spending $1 = 3 hours.
Check the video to see how the game looks.
The story is about you starting as a slave, gradually accepting you god legacy, and making choices about it.
Will you be a protective hero or a malevent one ?
The game does not judge you. 
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As for Baldurs Gate 3, Larian Studio developped it after the great success of Divinity Original Sin 2, it was so good that Wizards of the Coasts gave the licence to Larian Studio to develop the next Baldurās Gate.
While it is still in Early Access, I spent 160+ hours on it.
Check the 5th video which is the most representative of the story
https://store.steampowered.com/app/1086940/Baldurs_Gate_3/
The story is about you being infected by an Illithid parasite (process known as ceremorphosis). Your quest starts on finding a healer, you will meet lots of enemies and companions, and I suspect when the game will be fully revealed that the upper vilain (The Absolute) is an Illithid Elder Brain.
Baldurs Gate 3 is expected to be fully released in 1 year.
As Baldurs Gate 3 is still in Early Access, it is best to start with Divinity Original Sin 2 first.