For Mr. and Mrs. North, there's no vacation from murder In a remote cabin far from New York City, Jerry and Pamela North are getting killed. On the brink of annihilation, they grit their teeth and battle back. In a minute, the fight is finished--and the Norths are named mixed doubles champions. It's a happy moment during a splendid vacation, but off the tennis court, all is not well. Following an afternoon of fun and games, the evening's entertainment will be murder.   Mr. and Mrs. North have invited their closest friends--an ex-aviator, a mysterious doctor, and NYPD's own Lt. William Weigand--to join them on this glittering retreat, but the joviality ends when Weigand finds Helen Wilson lying across the path, a knife buried in her neck. A member of the group surely killed her, and unless the Norths act quickly, the murderer will strike again.