Shark turns upon an actual incident in WWII - mentioned in the film Jaws - when the ship which had delivered the fissile material to the south Pacific to be dropped on Hiroshima was subsequently sunk by a Japanese submarine with the loss of 900 men, including 200 killed in the largest shark attack ever recorded.
When the Creep, an American resident in the 1970s at the therapeutic community in north London supervised by maverick psychiatrist Zack Busner, starts to tell rambling stories of thrashing about in the water while under attack from sharks, Busner has to decide whether they are schizoid delusions or some sort of reality.