At the Shining Sea Aquarium, Dr. Karen James Madeleine Falk and two workers attempt to tag the resident octopus. The creature’s procedure triggers a cardiac arrest, then a startling reanimation: it lunges at Karen, takes the two workers’ lives, and slips away into the ducts, leaving the facility in a tense, unsettled hush.
Meanwhile, Miranda Riley [Eva Ceja] receives a cryptic transmission from Eden Wildlife Zoo, warning her to dispose of any epinephrine the aquarium received. Her colleague Daniel Hanley [D. C. Douglas] assures her the disposal is taken care of before he leaves to guide Skylar [Brandon Lee W] on a VIP tour through the building. Miranda then meets Senator Bailey Blackburn [Anthony Jensen], who is trying to secure more funds for the aquarium, but they’re soon interrupted by Karen’s urgent alert about the breach. They waste no time in notifying security chief Eddie “Clu” Cluwirth [Vivica A. Fox], who prepares his team for what’s coming.
Before any dramatic response can take shape, contaminated slime excreted by the octopus spreads through the tanks, turning the aquarium into a nightmare: the affected animals shatter their enclosures and stampede for safety. Miranda, Karen, Blackburn, and Blackburn’s aide Beth Mackenzie [Erica Duke] take shelter in a theater as lockdown locks the doors. Elsewhere, Daniel and Skylar scramble to get back inside and find themselves ambushed by a swarm of zombified spider crabs; they push through and manage to reach safety, a grim reminder that danger lurks in every corner.
As the group pushes forward, they deduce a possible way to lift the siege: reset the aquarium’s power to break the lockdown. They press onward, stumbling into the path of undead crocodiles and walruses as the facility vibrates with the uneasy rhythm of the crisis. Skylar’s nerves snap under pressure, and he is fatally attacked by a walrus. Karen realizes the reanimated animals are blind and rely on sound to navigate, a discovery she leverages by using her mobile phone to distract and steer the dangerous creatures away at crucial moments.
The danger intensifies when a starfish clings to Beth Mackenzie’s abdomen, having infiltrated her notebook earlier. The group works frantically to detach it, but time runs short, and Beth succumbs to the parasite’s feeding. They press onward through a perilous shark habitat, then descend into the basement where they finally confront the octopus itself. Clu makes a valiant but doomed attempt to kill the creature, sacrificing himself in the process, while Miranda works to reset the power.
With the power restored, the lockdown lifts and the survivors escape the chaos. The truth behind the outbreak comes to light: Daniel had schemed with Blackburn to cut costs at the aquarium and did not dispose of the Eden Wildlife Zoo epinephrine as claimed, which had been used on the octopus and triggered the outbreak. Miranda confronts Daniel, breaking his nose with a punch, and then she and Karen walk away together, leaving the betrayal behind and facing whatever comes next.
