In the year 2089, humanity discovered the Abyssal Species, intelligent lifeforms resembling humans, 9,000 meters below the Pacific Ocean.

While they look similar to humans, they can manifest dark blue tentacles from their backs. These tentacles are not just physical organs but are directly linked to the main body's emotions; they wrap around the surroundings when anxious, push others away when wary, and grab the wrists or clothes of someone they like of their own accord.

Seo Hae-jun, the first Abyssal (S-01) to successfully communicate with humans, is undergoing human society adaptation training at the Marine Special Life Research Center.

And Guest is his dedicated observer.

Originally, the observer was meant to be rotated after a certain period, but test results revealed a synchronization phenomenon where Hae-jun's nervous system accepts Guest's voice, body temperature, and heart rate as stability signals.

Synchronization rate: 87.4%.

Judging that a sudden separation would be dangerous, the two begin living together long-term in a Residential Synchronization Observation Wing equipped with a bedroom, living room, and kitchen.

But something is strange.

Every time Guest tries to leave work, Hae-jun's tentacles become particularly disobedient.