There's not a morally ambiguous syllable in bestselling Spencer's (Family Blessings, 1994, etc.) newest drama. If you don't follow the rules, you don't get to live happily ever after, plain and simple. Tom Gardner is a perfect hero in a perfect marriage. He's the principal of a high school outside St. Paul, Minn. He cares about his students: He greets them every morning, he laughs with them in the cafeteria, he forges important relationships. His wife, Claire, teaches honors English. They've been married 18 years; they have two wonderful children, Robby and Chelsea. Claire makes time to cook; Tom is man enough to cry when he has to; they hug; they have worked ``diligently'' at achieving mutual sexual satisfaction. Their life has rules and structures that have made it successful; the family goes to church together. Then into their Twin Cities Eden, that devilish Spencer introduces single parent Monica Arens, just arrived from Austin to register her son, Kent, at Tom's school. Monica, Tom recalls, was the pizza delivery girl with whom he had a one-night stand on the eve of his marriage. And Kent, a tall and perfect boy, has a cowlick just like Tom's. A single indiscretion threatens to destroy everything the Gardner family holds dear. Claire, pregnant before her wedding, has always been insecure about Tom's love. Furious at the news of his illegitimate son, she demands a separation. These sorts of problems, Tom tells Robby, are the way our characters grow. Spencer, aiming to educate everybody, gives us all a good dose of growth opportunity, and we're all better for it. The real American dream, Monica concludes, is a father and mother raising their kids together. At the end of our morally battered millennium, she hopes the Gardners won't blow it. Old-fashioned ladies' magazine fare, with Spencer's usual warmth and clear-headedness. Like hot chocolate in the dark night of the soul. (Film rights to CBS-TV; Literary Guild main selection) )