Into the Drowning Deep

In 2015, Imagine Entertainment sends the SS Atargatis to the Mariana Trench to film a mockumentary about mermaids. Communication is lost on May 17. The ship is found adrift six weeks later with no survivors and no bodies. Recovered footage shows gray-skinned creatures with eel-like tails, bioluminescent hair, and needle-filled mouths swarming the decks and killing the crew. One creature mimics the voice of Anne Stewart, an Imagine reporter aboard the ship. Imagine fights the footage's release, but the U.S. Navy leaks it, and the public dismisses the recording as a hoax.

Seven years after Anne's death, her younger sister Victoria "Tory" Stewart works as a marine biologist in Monterey, California, while pursuing graduate research at UC Santa Cruz. Tory has spent years using sonar to search for evidence of the creatures that killed Anne. Her research partner, Luis Martines, a cryptozoologist funded by his family's Silicon Valley fortune, brings her a breakthrough: deepwater sonar scans from near the Mariana Trench that match the sound of the Atargatis's engines, recorded at a depth where no ship could operate. Something in the deep is mimicking surface sounds.

That evening, Theodore Blackwell, a representative of Imagine, appears in their lab. He reveals that Imagine has been covertly supporting Tory's research for years and offers both scientists berths on the Melusine, a new research vessel bound for the Mariana Trench to determine what happened to the Atargatis. Tory agrees without hesitation. Blackwell also recruits Dr. Jillian Toth, the world's foremost expert on mermaids and Blackwell's estranged wife. Toth's theoretical framework holds that the creatures actively drove humanity from the ocean in antiquity, explaining why mermaid mythology appears in every human culture. She carries deep guilt for having provided the research that guided the Atargatis to the Trench while declining to sail with it, and she joins the voyage as a chance to atone.

The Melusine departs San Diego in August 2022. Key passengers include Olivia Sanderson, Imagine's on-camera reporter, and her cameraman and protector, Ray Marino; Jacques and Michi Abney, celebrity big game hunters hired as armed security; and three sisters: Holly and Heather Wilson, deaf identical twins specializing in organic chemistry and deep-sea submersible piloting, and their older hearing sister Hallie, an acoustician and sign language interpreter. Blackwell sails as Imagine's corporate observer despite chronic pain from a spinal injury, introducing himself to the assembled passengers as Imagine's onboard spy. Below decks, the ship hides three captive bottlenose dolphins intended to serve as scouts. Meanwhile, repeated tests of the ship's armored shutter system, designed to seal the vessel against external attack, all fail.

After anchoring above the Mariana Trench, Heather descends in her one-person submersible toward the Challenger Deep, the ocean's deepest point. She encounters a single mermaid, then hundreds more. They batter the submersible, the hull breaches, and Heather drowns.

That evening, Ray and Olivia are filming on the top deck when Olivia spots what appears to be a person climbing the hull. Ray offers a safety hook, but the figure is a mermaid. It swarms up the pole, bites into his shoulder, and drags him over the rail into the sea.

Blackwell releases the dolphins to scout, but the mermaids mimic the dolphins' own voices to lure them to their deaths. A mermaid enters the ship through the open dolphin hatch, and Blackwell traps it behind transparent titanium glass. Hallie approaches the tank and signs "Hello." The creature signs back, and communication between human and mermaid begins.

Tory, analyzing audio recordings, determines that the mermaids use three communication systems: signed language for close range, vocal sounds for distance, and mimicked sounds stolen from prey species as acoustic camouflage during hunts. When the creatures go silent on sonar, she concludes they are coordinating an attack.

Luis photographs a lone mermaid that climbs onto the deck, and Jacques and Michi shoot it dead, yielding the first specimen for study. Dr. Toth supervises a necropsy revealing the creatures are amphibians with dual breathing systems and a specialized lung that allows rapid transitions between crushing depths and the surface. Their larynxes contain structures evolved for vocal mimicry, a biological lure for intelligent prey. Toth reveals the specimen is male despite its feminine appearance: Curved ribs mimic breasts, and humanoid lips serve the mimicry apparatus. She declares these creatures are not "mermaids" but "sirens," whose resemblance to human women is an evolved hunting adaptation. Jason Rothman, Tory's ex-boyfriend and a plankton specialist aboard the ship, collects parasitic organisms from the dead siren's hair. While dissecting a pseudoshrimp, a shrimp-like parasite, in his cabin, he pricks himself on its venomous stinger and bleeds to death as the toxin destroys his blood's clotting factors.

As Tory and Olivia grow close and confess mutual feelings for each other, the sirens launch a coordinated nighttime assault, scaling the hull in waves and attacking in silence. Scientists and crew are killed throughout the ship. Michi is struck by friendly fire: A bullet passes through a siren's body, carrying the creature's toxic mucus into her bloodstream. Anticoagulants and neurotoxins in the mucus destroy her blood cells, and she dies roughly an hour later.

Trapped in Toth's lab with Luis, Holly, and the incapacitated Blackwell, the group devises an escape plan. Olivia volunteers to climb the ship's narrow internal service tubes to the control room, arguing she is the smallest, fittest, and least essential to the science team. Tory escorts her to the hatch, and they share a brief kiss. Turning back, Tory encounters a juvenile siren, runs, slips on mucus, and falls overboard. Surrounded by sirens in the water, she is ignored; the creatures are too focused on their assault to notice her. Looking down, she sees an enormous shape rising from the depths, outlined by bioluminescent glow: a female siren, hundreds of times larger than the males. She realizes the species exhibits extreme sexual dimorphism, like anglerfish. The males hunt to feed the matriarch, who consumes everything. This explains why the Atargatis was lost with no bodies.

Olivia reaches the top deck and finds two surviving engineers. The captain is dead and the distress beacon was never activated. Olivia activates it, calls Blackwell for the shutter code, and deploys the shutters, sealing the ship. Sirens are knocked from the hull, and those trapped inside begin suffocating in the open air. Tory pulls herself back aboard through a water intake port on the ship's underside and nearly drowns behind an internal barrier before Hallie wrenches it open and drags her to safety. Tory phones Olivia and orders every light turned to full power, driving the rising female siren back into the depths.

In Blackwell's private lab, the captive siren signs to three intruding sirens, which withdraw without attacking, protecting Hallie and Dr. Daniel Lennox, the cetologist who communicates with the dolphins. Jacques, upon learning of Michi's death, goes on a solitary rampage through the ship, killing over thirty sirens before being overwhelmed and killed.

Two days later, the naval vessel USS Datlow arrives, finding the Melusine covered in slime and puncture marks. Sailors discover blood-soaked corridors, dead sirens, and traumatized survivors. In an epilogue, Jim Alway, a marine animal entrepreneur in Hawaii, receives a call confirming the Melusine will dock in six hours with the captive siren aboard. He prepares his team, anticipating profit from the creature. Two hundred miles offshore, Tory and Olivia lie tangled together, asleep, both dreaming of the voyage: Olivia runs through a ship of ghosts, and Tory swims through frigid water, drawn onward by the bioluminescent light of the sirens.