Couple very much in love and a son. All working for agency. Submarine is sunk. Wife on it. Husband investigates. Son kills him. Or does he. He disappears. No body. People trying to find husband. Giant 3 meter high cake.
Underwater array of sensors. Sub goes over it. Not detected. Followed by a little sub, a watcher. Original sub was detected (in port phillip bay) because of a fault. Watcher sub picked up by hand and pushed through the water.