It's 1966 and Queen Elizabeth II is touring the Caribbean aboard the Royal Yacht Britannia. Her schedule is measured in ten-minute increments for five weeks, and nothing can afford to go wrong.

But something does. Early in the trip, the Queen's new diary secretary, Pamela Sinclair, is found dead in her cabin. Rumours abound about how she died, but the truth is, nobody really knows. However, this time, suspicion falls on the Queen's longstanding confidante, Joan McGraw.

The royal yacht is a busy, working ship, from the admiral at the bridge to the chefs in the galleys and the engineers attending to the turbines. It houses a crew of hundreds. Is one of them a murderer?

The Queen and Joan, must use every skill and trick to solve the mystery while seeming to glide, unflustered, from one be-hatted engagement to the next, as the multi-island visit unfolds with the whole world watching.