When Forbes Ingraham, a respected lawyer, turns up dead in his office, the main clue is w a gold key that allows access to the back door of the law firm of Schaeffer, Ingraham and Webb. The key had belonged to Schaeffer, also deceased, and had been given to him by his wife. But what was it doing in lngraham's office and, more importantly, who had used it? Enter Pam and Jerry North, friends and clients of the late Mr. Ingraham. With the secretaries all in a flutter and a tangled web of relationships, it is not easy to figure out what happened — especially since gangsters seem to be involved through a noted client of Mr. Ingraham. But leave it to Pam's ''woman's intuition'' to put together the puzzle pieces and make sense of the key clue.