A mushroom trip brings free-spirited Elliott face-to-face with her 39-year-old self. But when Elliott's "old ass" delivers warnings to her younger self, Elliott realizes she has to rethink everything about her family, life and love.
Elliott drives her little boat across a lake with her best friends, Ro and Ruthie, aboard. Her family waits at home to surprise her with a birthday cake to celebrate her 18th birthday, but Elliott has other plans. She stops at a coffee shop where Chelsea, with whom she has flirted since 8th grade, works. She reveals that she is leaving for college in Toronto soon, and the two finally have sex while her friends chat outside. When Elliott returns, Ro reveals that she brought along hallucinogenic mushrooms for the trio to take while camping out overnight.
The friends motor across the lake to the woods, set up a tent and brew the mushroom tea. Ro and Ruthie trip out happily. Just as Elliot says that she feels no effect, a woman appears next to her and says she is her older self. The woman shows a scar on her rib cage and knowledge of Elliott's asymmetrical breasts to prove that she is Elliott from the future. The 39-year-old Elliott mentions that she is a PhD student. Elliott begs for some life-changing advice, and older Elliott suggests bonding with her family, especially since she will soon be leaving for college, and mysteriously tells her to avoid a boy named Chad. The pair go to sleep in the tent, and older Elliott puts her number in younger Elliott's phone under the name "My Old Ass". Elliott wakes up as Ro and Ruthie are packing up the boat. Later at home, Elliott is impatient with her mom but expresses remorse when she finds out about the birthday surprise.
The next morning, she continues her affair with Chelsea and is late to work on her family's cranberry bog. After working hard, Elliott goes skinny-dipping and encounters a boy swimming nearby. He works for the summer for Elliott's father at the bog and reveals that his name is Chad; Elliott runs away. Alarmed that she finds him attractive, Elliott texts "My Old Ass". Initially unnerved that she receives a response, she throws her phone into the lake. Elliott eventually goes swimming again and sees Chad. She watches him work but fears that her attraction is growing and again runs away. Elliott tells her older self that she spoke with Chad and that he seems harmless, and she seems to have found him attractive. Older Elliott refuses to say what the problem is with Chad.
Older Elliott disappears for four days, but Elliott takes her advice, avoiding Chad and spending time with her brothers, Spencer and Max. She finds common ground with Max while watching him play golf. One morning, Elliott finds Chad at the family breakfast and tries to escape with Max, but Max invites Chad to come with them to try golfing. Max reveals that the family is selling the farm. Elliott hurries to confront her dad, who says that he did not think she cared about the farm.
While trying to sell her boat, Chad and Elliott kiss after a conversation, causing Elliott to freak out and flee again. Unable to reach her older self, Elliott gets the rest of the drugs from Ro, who wonders why she has been missing lately. During her mushroom trip, she does not see older Elliott but hallucinates that she is Justin Bieber, performing "One Less Lonely Girl", and gives Chad red roses. The next day Elliott talks to Ro about possibly being bisexual and not a lesbian as she had thought. Later, she talks with her mom about her childhood and tells her that she will miss her when she leaves for college and will always need her. They reaffirm their love for each other. Elliott soon introduces Ro and Ruthie to Chad and goes on one last boat ride with him, but the motor falls off the boat mid-ride as it starts pouring. The pair get to a lookout shack, and Elliott tells Chad how she feels and discovers he already attends her college. They have sex.
After Chad walks Elliott home Older Elliott reappears. Older Elliott reveals that Chad will die and that she cannot save him. She does not want younger Elliott to experience the same pain. Chad returns to bring Elliott her shirt and can see older Elliott. The two awkwardly introduce Older Elliott as Elliott's "uncle" Michelle, and Older Elliott hugs Chad, weeping. She says goodbye and leaves forever, now encouraging Younger Elliott to experience this tragic love. Elliott receives a voicemail from older Elliot telling her not to live for the past or future, but for herself in the present. Chad has fixed her broken boat, and Elliott takes it on one last ride, saying goodbye to the lake.
