You can get a headache trying to think about The Interestings on a straight timeline, so let us break it down for you.
First, we get a glimpse of our main cast as teenagers at Spirit-in-the-Woods, the summer camp that ends up being the formative moment for a lot of what happens later. The year is 1974, and Julie, Ash, Goodman, Jonah, Cathy, and Ethan are typical teens with raging hormones, varying artistic talents, and vague dreams of "being someone" in the future. The thing you need to know the most? This is where Julie becomes Jules and falls in love with/totally idealizes the group that calls themselves the Interestings.
Next up is the same group—a couple of years and several different romantic pairings later—when they first start to fall apart. Goodman is accused of raping Cathy, his on-again-off-again girlfriend, and the Wolf family totally doesn't believe it and pays tons of money to get him acquitted. Goodman pulls a typical Goodman, though, and skips out on the trial. Oops.
The thing you need to know the most? This is the heart of the ambiguity and deception in the book. No one can ever decide whether Goodman did in fact rape Cathy, and since he flees to Iceland and his family secretly visits him, the big ol' Goodman problem is a source of tension for the rest of the novel.
In between the major events here, Jules and Ash marry Dennis (the only significant non-Interesting character) and Ethan, respectively. Ethan creates Figland and becomes a famous animator, while Ash becomes a semi-successful playwright and director in small, off-Broadway theaters. Jules becomes an average therapist after failing at being a comedic actor, and Dennis is an ultrasound technician with a history of depression.
Oh, and Jonah is… something. He sort of disappears from the story for a while, comes out as gay, we get a drug-fueled backstory, and he has a brief, cautious relationship with an HIV-positive lawyer. Cathy is there, too, somewhere, but she also kind of disappears after the Goodman issue.
To round out the trifecta of main plot points, we get a sudden, super dark turn toward depression, nearly-fatal accidents, cancer, and mental disabilities. No, really: Brace yourselves.
Ash and Ethan have a son who is diagnosed with autism, and Ethan struggles for the rest of the novel with his inability to love his child. The only drug that works for Dennis's depression reacts with a food he's not supposed to eat and he nearly dies, so the doctor takes him off the antidepressant and he goes into a downward spiral that almost destroys his marriage with Jules. And topping it all off is the abrupt revelation that Ethan has skin cancer, and the equally abrupt series of chemo appointments that lead to his death. Wow… that was a downer. The end.