This is the beginning.
Only chapter 5 is part of the series, so skip the first 4 chapters.
Hydra revolves around Utsumi Ryuu, a teenager with a dark past that tends to transfer schools a lot. He is the core of Hydra and the reason the story exists in the first place.
TW for a short, but uncomfortable adult - minor relationship. It's also the only one in Hydra, don't worry about inappropriate stuff anymore.
TIMELINE
A very young Ryuu (16) who has to deal with fresh trauma.
This and Kiss After can be skipped. This is pretty much an interlude.
School transfers timeline: Ryuu's dark past happened in Osaka. He moved to Okinawa (Please Chapter 5), then moved again off-screen to the city in Kiss and Kiss After where he had a short relationship with Sakai, then once again moved and transferred to the school in Hydra.
TIMELINE
It takes place about half a year or a year after Please. Ryuu is about 17 here.
Sequel to Kiss.
Again, this and Kiss can be skipped, but if you want to spend more time with Ryuu and understand him better as a person before we move to the main story, read them.
TIMELINE
It takes place about half a year or a year after Please. Ryuu is about 17 here.
Bonus content. This introduces three new characters: Itou Yuuji, Taira Hikaru and Hitomi Naoyuki, that have significant importance in the main story.
You'll understand their relationships and roles while reading.
TIMELINE:
This happens a few days before Hydra officially starts, with Taira as the protagonist, Itou as a side character and Ryuu and Hitomi as cameos.
Ok, here we go. This is the main story. Everything you've read before this was bonus, the build-up, getting introduced to Ryuu and his demeanor.
Read chapters 1 to 6 from this, then go to Hydra dj - Heavenly ONLY if you like Taira. If you're not particularly interested in him, you can read till chapter 9.
TIMELINE:
We're about a year and a half or 2 years after Please.
Ryuu is in his final year of high school and almost 18.
Bonus content. Very short. Can be skipped if you're not interested in Taira, but you should read it if you are.
Now go to chapter 7.
Read chapters 7 to 9 in one go, then go to Blue Film First ONLY if you like tairatomi as a couple. If you don't, you can continue reading until the end (chapter 11).
Bonus content. A must read for tairatomi enjoyers.
I recommend reading it between chapter 9 and 10.
If you're not particularly into this pair, you can skip this one and go to chapter 10.
This is the end of Hydra.
Congrats! You have completed the main story of Utsumi Ryuu, Hitomi Naoyuki and Taira Hikaru.
Bonus content.
This dj takes place before the second half of chapter 11. So it kinda takes place right in the middle of it. It's skippable, but if you're a fan of the main couple and like Ryuu as a character, you want to know what he's thinking for once, definitely worth reading.
In unofficial orders people will tell you to read this before chapter 11, but don't, as it will spoil the finale. I think it's better to read it after you finish the entire story.
Please (Miyamoto Kano)