Seventeen-year-old Chloe Mitchell is a high schooler in Los Angeles whose twenty-year-old sister Ivy is autistic. When Ivy starts asking Chloe questions about kissing and sex, Chloe decides to set her up with Ethan, a boy from Ivy's special needs class. Chloe discovers too late that her rude classmate David Fields is Ethan's younger brother. As Chloe, Ivy, Ethan, and David get to know each other, Chloe considers her own romantic choices.
Things I Should Have Known contains examples of:
- Brilliant, but Lazy: David spends every class browsing the internet and only participating to disagree with other people. He gets such good grades that the teachers let him do whatever he wants.
- Calling Parents by Their Name: Chloe calls her mom "Jeannie" while talking to other people, for reasons she's unsure of.
- Character Tics: Ivy bangs her hands against her thighs or hips when she's upset.
- Does Not Drive:
- Ivy can't drive. Since Ron expects her mom to work for free as his receptionist, Ivy relies on Chloe to drive her to and from school, since the short bus is too loud, as well as to her dates with Ethan.
- Ivy's classmate Diana once drove her father's car in a parking lot, but it was scary and he yelled at her a lot, and now she isn't sure she wants to drive even though Ivy thinks she'd be good at it.
- Go to Your Room!: After Ethan runs off from Chloe's house, wanders around all night, and is possibly mugged, leading Margot to decide to institutionalize him, Margot forces him to stay in his room all day so it doesn't happen again.
- Incompatible Orientation: Ethan really likes Ivy, but Ivy doesn't like him back because, as she realizes, she's gay and in love with Diana. When Ethan finds out, he's so devastated that he runs away and isn't found until late that night. Ivy is on the receiving end the next day when she tries to kiss Diana, and Diana says that gay people aren't normal.
- Interrupted Intimacy: Chloe and her boyfriend James are making out on the couch when Ivy walks in and turns on the light, startling Chloe so much she falls on the floor.
- Missing Mom: David and Ethan's mom left after Ethan was diagnosed. Her explanation was that she'd always felt there was something wrong with her husband, and since it was his "fault" their kid was disabled, she shouldn't have to deal with it.
- Parent with New Paramour:
- Chloe and Ivy's stepdad Ron, who's been married to their mom for a year. Chloe dislikes his forced efforts to act like their dad and especially his controlling behavior towards Ivy, whom he nags about her diet. She starts to warm to him when he stands up for Ethan's right to stay with his family instead of being institutionalized.
- David and Ethan's stepmom Margot used to be okay until she got pregnant and went off the deep end. Now she's an anti-vaxxer who's obsessed with the possibility that her son Caleb will turn out to be autistic and is constantly pushing for Ethan to be institutionalized.
- Portmanteau Couple Name: In-universe example. Chloe wonders if she should call Ethan and Ivy "Eth-vy" or "Iv-an."
- Sleepy Depressive: After Chloe and Ivy's dad died about five years ago, their mom often wouldn't get out of bed in the morning. The sisters had to get ready for school by themselves.
- Too Unhappy to Be Hungry: After Chloe finds out that Ethan is going to be institutionalized, she's so upset that she skips dinner.
