I’m sitting on the beach, watching my son play in the water, and thinking about Gina, my boyfriend’s other girlfriend.

I imagine her walking down the street wearing a turban and a loose-fitting dress. Nobody suspects that this pretty woman is bald and missing part of a breast. Somewhere in her body, a few stray cancer cells that escaped the surgeon’s knife may be floating around, impossible to detect. The grains of sand on this beach are as numerous as the cells in a human body. I think of how hard it would be to search out and destroy just a few “bad” grains.

When I first started dating Ted, he told me about Gina, but not much, which was fine. He obviously wasn’t interested in seeing her exclusively, so I figured anything could happen between us — maybe even true love. That was three years ago. Since then, I’ve learned that Gina has been married twice and is childless, owns a chic women’s jewelry shop, had a nose job when she was younger, and made the striped curtains that hang in Ted’s kitchen. Chances are, Gina has gleaned some information about me, too. She probably knows I have never been married but have one son, and that I gave Ted the Klimt print that hangs in his bedroom. I wonder if she knows that I write for a living, and that I dye my graying hair. I wonder if she, too, has resigned herself to the fact that Ted has no monogamy genes.

As I shiver in the chill wind, my eleven-year-old son does cartwheels in the tide. He wears a bathing suit; I wear a jacket and wide-brimmed hat to protect myself from the last rays of sun. Wrinkles are even more frightening when you don’t know who has more of them, you or the other woman.

The apples and juice we brought with us now seem unappealingly wholesome. What I crave is a glass of brandy in front of a roaring fire — ideally, with a lover who wants only me. But then I always yearn for what I don’t have: milky skin, Katharine Hepburn’s neck, a rapier wit, a committed man.

My dog cavorts alongside my son, who runs down the beach trailing a kite that rises slowly behind him. It floats high above us, red and white and gold against the late-afternoon sky. My son comes to sit beside me, and attaches the kite string to two sticks he’s placed upright in the sand. “Look at my invention!” he says. I envy how he stays with the moment. The kite has risen far above us, its string stretched taut.

Gina and I just happened to fall for the same guy — a man who married the wrong woman, was miserable for twenty years until the divorce, and now wants to answer only to himself. Ted once told me he thought Gina and I would like each other, that we share many of the same values. We seem to have at least one thing in common: an ability to make do with what we’ve got. We’re two pragmatic women in our forties, no longer holding out for the “one and only.” Instead, we’re settling — possibly for less than either of us deserves; possibly for more than either of us would otherwise have.

Who knows? In another life, maybe she and I would become friends. She could teach me to wear jewelry with flair; I could let her share in the joys of motherhood. But in this life, I often wish Gina would drop off the face of the earth — like last week, when I saw a bottle of champagne in Ted’s refrigerator and knew, since he doesn’t drink, that it must be hers. (Of course, if Gina ever reached behind the frozen waffles in his freezer, she’d find my vodka and feel the same pang of jealousy.) But how can I be jealous of a woman who recently had a tumor removed from her breast, and is now suffering through chemotherapy and radiation? It would be unthinkable to begrudge her time with her man, even though he’s my man, too.

Last night, as Ted and I were leaving for a movie, Gina phoned, feeling panicky and depressed. Ted promised to call back when we returned. He did, while I waited in the bedroom. They spoke for just five minutes, but I think he helped to calm her down.

My wild, windy stretch of beach is surrounded by cliffs. Behind me, tall reeds of grass grow in a circle, forming a clearing. When I was little, I would have made a hideout of that space. I would have let the grass shield me, keeping me safe.

My son has let go of the kite string, and it falls onto my skirt. He is standing at the water’s edge, daring the waves to come closer. The undertow is powerful here, so I keep careful watch. His presence anchors me to what’s real.

Like it or not, Gina is real. The baking pan I saw on Ted’s counter last week was probably hers; I suspect she made a cake for his birthday. The casserole dish beside the pan was mine; I made strudel. I close my eyes and try to visualize this woman I’ve never met, with whom I share so much. For a moment I stop wondering whether she would outshine me on the dance floor, upstage me at a party, outdo me in the kitchen or the bedroom. Instead, I envision her standing in the middle of a tall ring of grass, protected from the wind. I may have wished she would disappear, but I’ve never wished her ill.