Unexpected happenings in “The Summer Everything Changed,” the new novel by Holly Chamberlin, unfold like the escape of evil from Pandora’s box. That’s the good part of the book.

What’s less appealing is the pace of this long, 400-page mystery. Brevity would have improved it.

Nevertheless, “The Summer Everything Changed” is a chilling and realistic read about a young woman blindsided by an abusive male. The novel is also admirable for the way Chamberlin can switch settings from happy-go-lucky to very dark.

Her book’s opening reveals few hints of events to come. Good-hearted Louise Bessire is owner and operator of a picturesque Ogunquit hostelry called Blueberry Bay Inn. She appears to have a great relationship with her 16-year-old daughter, Isobel, a creative and seemingly well-adjusted young woman.

With a skeleton crew of likable people, mother and daughter keep up the inn. At this early point in the book, their biggest challenge for the upcoming summer is a looming celebrity wedding made difficult by a demanding wedding planner.

The book appears at this juncture to promise a fluffy read about (yawn) the ups and downs of running an inn in Maine.

There are some hints of darkness. Louise was once abused by a college boyfriend who pushed her down a flight of stairs, causing serious injury and the death of a baby she was carrying.

But that’s in the past. Now she has Isobel, an outgoing girl who maintains a surprisingly popular blog she calls “CityMouse.” Isobel has a close friend named Gwen, a happy adopted child of two married fathers.

“The Summer Everything Changed” shifts gears when Louise meets (by literally bumping into) mysterious Jeff Otten. He’s the youngest son of Ogunquit’s wealthiest couple.

Jeff is very handsome. He’s about 20. Townspeople assume he’s going to college somewhere or working for his father. But strangely — considering the Otten family’s wealth and curiosity of neighbors — nobody knows much about him. At least, nothing they’re willing to talk about.

Isobel is flattered by Jeff’s attention, and surprised when he gives her a diamond bracelet only days after their meeting. Louise thinks Jeff is a polite young man and a catch for her daughter.

But some are uneasy. Gwen is troubled by the mysterious Jeff, who — sensing her ambivalence toward him — dislikes her intensely, and seeks to drive her away from Isobel.

And a strange happening on a sidewalk in Ogunquit occurs when a dog that’s friendly toward everyone growls menacingly at Jeff. Isobel’s initial embarrassment for Jeff turns complicated when she sees the hateful expression in his eyes as he glares at the small dog.

From about that point on, the novel morphs into an upsetting tale with interesting twists and discoveries. Isobel, who shared the stage with her mother early in the book, becomes the central character standing alone. Her remarried father is far away and too distracted with his new wife to help his daughter. Isobel has alienated herself from Gwen. Her mother has the celebrity wedding to pull off.

Chamberlin reveals her characters insightfully. Isobel, for instance, writes her “CityMouse” blog in the persona of a cheerful teen on a perpetual shopping trip through southern Maine. But as her life falls apart, maintaining her blog persona is tough.

“Isobel closed the laptop and sighed deeply,” Chamberlin writes. “She felt tired and more than a wee bit grumpy. Faking exuberance wasn’t as easy as some people made it out to be.”

Chamberlin is a seasoned author of 10 books, and has also collaborated with other writers. Her novels, among them “Summer Friends” and “The Family Beach House,” have been singled out for praise. A native of New York City, she was a book editor and writer in Boston before moving to Portland with her husband.

While I enjoyed the quick plot shifts of Chamberlin’s latest novel and admire her storytelling skill, her latest book would have more punch with fewer pages. Also, it takes Louise Bessire a surprisingly long time to recognize that her daughter is in deep trouble, especially in light of her own experience with an abusive male.

But overall, “The Summer Everything Changed” is a good read and a memorable novel.

Lloyd Ferriss is a writer and photographer who lives in Richmond.