Magnus and Will set out on a short underwater mission to respond to a distress call from a peaceful abnormal species deep below the Bermuda Triangle. While sending out specialized sonar signals to elicit a response from anyone who might be within range, they discover a grizzly massacre of the half-human abnormals who once lived in the waters.
Bewildered by who, or what, could cause such devastation upon a peaceful species, Magnus retrieves one of the creatures from the water to take back to the Sanctuary for examination, hoping it will shed some light on what could have occurred.
Magnus and Will begin their ascent. Still slightly in awe of Magnus and all she has seen and done, Will seizes the opportunity to learn more about her adventures ...and acquaintances ...over the years.
As Magnus checks the submarine's rate of ascent, she experiences a painful twinge that rapidly increases in intensity. From his bunk, Will hears a crash and rushes to find Magnus collapsed on the floor in excruciating pain. Wracked by inexplicable pressure from the ascent, she demands they take the sub deeper as the only way to alleviate her agony.
Once stabilized, Magnus and Will run blood tests in an attempt to determine what is causing her inability to withstand the ascent, but before long, she starts feeling dizzy again. Through the pain, Magnus becomes uncharacteristically aggressive and orders Will to take them deeper. They both know that the sub can only sustain a mid-range depth before crushing under the pressure ...and killing them both.
With time an imminent threat, they are shocked to find a hostile parasite that is rapidly taking over Magnus's body and mind.
Knowing that ascending will surely kill Magnus and descending will ultimately kill Will, they are faced with an impossible dilemma: find a cure for Magnus's deadly condition, or sacrifice her as the host.