George, while on patrol with a parking officer, witnesses a car drive away from a dead body. He sees two men in the car, a driver and a passenger. These two men are then prosecuted for murder, and George is called to be a witness. The passenger's brother, who is a friend of George's girlfriend, asks George not to testify at the trial. He threatens George and tells him to "watch [his] back" if he does decide to testify. Later in the story, George is murdered while on his surfboard out at sea. The murder seems staged to look like a shark attack. This is strange because the idea of staging a murder to look like a shark attack is an idea that George, a writer, had for a story—an idea which he only told to his girlfriend.
The implication at the end of the story is that the passenger's brother killed George, or had him killed, to prevent him from testifying against his brother. The passenger's brother very likely got the idea for the murder from George's girlfriend, as she, besides George, was the only person to know about the idea. One implication in the story is that George's girlfriend is possibly having an affair with her friend, the passenger's brother, and thus gave him the idea for the murder. Staging the murder as a shark attack is a good method for the murderer to choose, because shark attacks have been known to happen in the area, and so the death will be accepted as a shark attack.
The identity of the driver, at the end of the story, remains mysterious. The driver is unlikely to be the passenger's brother, because if he were, George would have recognized him as the driver when he approached him to ask him not to testify. George saw both men in the car. Whoever the driver might be, he would likely also want George killed to stop him from testifying against him at the murder trial.