Miri works as a grant writer for nonprofit organizations and lives in a constant state of anxiety. Her nervousness predates her wife's departure but worsens significantly during the long, unexplained absence. Left in the dark by her wife's secretive employer, she takes on the role of caretaker for a suddenly uncommunicative partner. She tries to reconcile her nostalgic memories of their early romance with the disturbing physical reality of the present.

Leah is a passionate marine biologist and researcher employed by the Centre for Marine Enquiry. Typically grounded and intensely curious about the ocean, she returns from a routine three-week mission that inexplicably stretched to six months. Now withdrawn and suffering from strange physical ailments, she spends her time retreating to the bathtub, silently reflecting on her terrifying isolation trapped at the bottom of the sea.

Jelka is a crew member on the Centre's deep-sea mission. Trapped in the dark ocean depths when the vessel loses communication and control, she relies heavily on her religious faith to cope with the isolation. She brings along a figurine of Saint Brendan of Clonfert and turns to constant prayer as the days stretch on in total darkness.

Matteo is the third crew member on the Centre's deep-sea expedition. As the submarine remains stuck at the bottom of the ocean, his frustration grows rapidly. He becomes increasingly agitated by his lack of control over the situation and highly skeptical of the organization's true motives for sending them down.

Carmen is Miri's friend who suffers from a degenerative eyesight condition. She attempts to offer Miri sympathy and normal conversation during the difficult readjustment period, though she struggles to grasp the true severity of what is happening in her friend's marriage.

Juna is Jelka's sister. She shares an uneasy connection with Miri through their respective family members' employment at the Centre for Marine Enquiry. She experiences her own frustrations dealing with the bureaucratic stonewalling of the secretive organization.

Miri's deceased mother casts a long shadow over her daughter's present life. Described as rigid and highly paranoid about health risks, she maintained an emotionally distant relationship with Miri. Her memory constantly reminds Miri of the negative traits she fears inheriting.

Leah's deceased father had a harsh, unorthodox approach to parenting his daughter. Despite his rough methods and their complicated dynamic, he inadvertently sparked Leah's deep, lifelong fascination with water and the ocean.

Pamela is an octopus that Leah befriended while working at an aquarium during her youth. The distinct bond she formed with the sea creature led to a lifelong collection of octopus-themed postcards and trinkets.