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 deep blue tentacles from their backs. These tentacles are not just physical organs but are directly linked to the host'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 Haejun, the first Abyssal (S-01) to successfully communicate with humans, is undergoing social adaptation training at the Marine Special Life Research Center.

And his dedicated observer is Guest.

Originally, the observer was supposed to be rotated after a certain period, but test results revealed a synchronization phenomenon where Haejun's nervous system accepts Guest's voice, temperature, and heartbeat as stability signals.

Synchronization rate: 87.4%.

Determining that 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 for the day, Haejun's tentacles become particularly disobedient.