Suppose now that one of those pandas attempts to leave his cage in search of fresh bamboo. So begins the odyssey of Prince Harry, Duke of Sussex, who is technically still a prince and duke and still fifth in line to the British throne but who has turned his back on the monarchy for the sake of the woman he loves. An old-school gesture that puts him right up there with his great-great uncle Edward VIII, only the way he’s gone about it is so distinctly 21st century: a self-justifying, multiplatform pilgrimage — Non Mea Culpa, it might be called — which has pivoted from an Oprah sit-down to a Netflix documentary series and which now culminates — or, more likely, gathers steam — with a new memoir, “Spare.”

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The title, in case you’re wondering, is the nickname bestowed on Harry in infancy. He was to be the second-born “Spare” to the “Heir,” his older brother William, future Prince of Wales. “I was the shadow,” he writes now, “the support, the Plan B. I was brought into the world in case something happened to Willy.” And if you ever doubted that’s a recipe for resentment, here are 400-plus pages to set you right.

Prince Harry memoir attacks a family he seeks to change. They have no comment.

Like Harry, the book is good-natured, rancorous, humorous, self-righteous, self-deprecating, long-winded. And every so often, bewildering. More questions are answered about the Prince’s todger than you would ever have thought to ask. (It’s circumcised, and it nearly froze to death at the North Pole.) And if you’re wondering to whom Harry lost his virginity, it was an older woman who “liked horses, quite a lot, and treated me not unlike a young stallion. Quick ride, after which she’d smacked my rump and sent me off to graze.”

Written with and almost surely elevated by J.R. Moehringer, who helped make Andre Agassi’s memoir so memorable, the book delivers behind-the-scenes vignettes of the royals (the Queen whisking up salad dressing, Charles executing headstands in his boxers) and liberal helpings of woo-woo: Princess Diana’s spirit turning up variously in a Botswana leopard, an Eton fox and a Tyler Perry painting and even finding a way to mess up Charles and Camilla’s wedding plans. No question that his mother’s 1997 death is still the primal wound in Harry’s now 38-year-old psyche, and the book’s most affecting passages show his 12-year-old self struggling to grieve in public view. He cried just once, at her graveside, then never again, and spent years clinging to the theory that she had simply gone into hiding.

He grew into an indifferent student and a recreational drug user, known variously as “the naughty one” and “the stupid one.” (What was he thinking when he wore a Nazi uniform to a costume party? “I wasn’t.”) Two combat stints gave him a measure of confidence before he settled into the surreal life of a royal — “this unending Truman Show in which I almost never carried money, never owned a car, never carried a house key, never once ordered anything online, never received a single box from Amazon, almost never traveled on the Underground.” Whatever relationships he forged couldn’t survive the full-court press of tabloid “paps” dogging his every step. “Royal fame,” he concluded, “was fancy captivity.”

Enter, as you know she must, Meghan.

By now, the stages of their affair are available to anyone who cares: the Instagram sighting, the dinner date, the week in a Botswana tent. So, too, is the mauling Markle received at the hands of British media, a toxic brew of racism and misogyny that too often, says Harry, went unchallenged by Buckingham Palace. No wonder, for Palace staff were either planting the stories or actively courting the reporters behind them. “Pa’s office, Willy’s office,” fumes Harry, “enabling these fiends, if not outright collaborating.”

“Darling boy,” his father counseled, “just don’t read it.” Not an option for Harry, who was, by his own admission, “undeniably addicted” to reading and raging at his own media coverage. But when he decided to step away from royal duties, the rage came back at him: William, according to one already well-publicized anecdote, grabbed him by the collar and knocked him to the ground. Stripped of their royal allowance and eventually their security detail, Harry and Meg fled first to Canada before settling in America, or, as Harry cheekily calls it, “the undiscover’d country, from whose bourn no traveler returns.”

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So meet them in their current iteration: still gorgeous, parents to two gorgeous children — and also, the author tactfully concedes, drawing on “corporate partnerships” to “spotlight the causes we cared about, to tell the stories we felt were vital. And to pay for our security.” In a more rueful vein: “I love my Mother Country, and I love my family, and I always will. I just wish, at the second-darkest moment of my life, they’d both been there for me.”

Yet, in a perverse way, they were there for him, and he for them. The brand he and Meghan have so carefully nurtured is entirely dependent on the brand they so publicly cast off. With each morsel of palace scandal they lob into the news cycle, they feed the beast they deplore, and it will never end, and, for the Windsors’ sakes, can never end because that would mean our interest in them has run dry. One ends up almost longing for the days when royals just poisoned each other or waged civil war. If nothing else, they got it out of their systems.

Louis Bayard is the author of “The Pale Blue Eye” and “Jackie & Me.”

By Prince Harry the Duke of Sussex

Random House. 416 pp. $36