Off the coast of Mexico, Dr Susan McAlester's (Saffron Burrows) team of scientists are working on a cure for Alzheimer's disease by injecting degenerate human brain cells into mako sharks. The three sharks selected grow to over forty feet in length, and begin to demonstrate signs of intelligent behaviour. When pharmaceuticals' president Russell Franklin (Samuel L. Jackson) arrives with funding in mind, one of the sharks attacks scientist Jim Whitlock, biting off his arm. However, the helicopter taking Whitlock to surgery crashes into the research centre and disables the protective security systems which keeps the killer sharks at bay...