With you, I could drown in Eden

"Sentinels" cannot live without their "Guides"— Koudai, an ordinary high school student, reunites after several years with his childhood friend Shiro, whom he once had feelings for.
Their joyful reunion is short-lived when Koudai learns that Shiro is a "Sentinel" living in the underworld, and he himself is a "Guide," a person with supernatural abilities.
As a price for their heightened senses, Sentinels face life-threatening danger unless they physically connect with their Guides. Shiro is no exception, and he demands sex from Koudai...?!
"If it means letting others touch you, then I’ll—" As they give in to their physical desires, special feelings gradually begin to grow. Koudai starts to desperately wish to save Shiro from the underworld, but...