Detective John Steel is back in Britain searching for clues to the organisation who murdered his family.
He finds a name - "Neptune". "Neptune", he discovers, is the name of a cruise ship about to take its maiden voyage. Steel books passage on the ship, determined to find out what is so special about the ship, and what it has to do with his family's killers.
Meanwhile, in New York, Steel's partner, Detective Samantha McCall, and her team investigate what appear to be five random homicides. Soon Sam realises they are all connected, and that one person may be responsible. Is there a link between the killings and the deadly discovery Steel has made aboard the cruise ship? Will Steel and McCall be able to share their findings and foil the plot before it's
too late?