Louisa shows up at the auction for a valuable painting called The One of the Sea. She is disgusted by how the rich people at the auction seem to only care about the painting’s monetary value. When a guard catches her with spray paint, everyone assumes she is an activist there to deface the painting, so they chase her out.
While Louisa is fleeing the auction, she bumps into someone whom she believes is a homeless man. In fact, it’s not a homeless man but C. Jat, the very same artist who painted The One of the Sea. Louisa recognizes his identity after the two of them paint graffiti together, and she notices that he paints skulls just like the signature on The One of the Sea. Louisa has had a difficult life and is happy to meet a kindred spirit like C. Jat. After her parents died when she was young, she spent a lot of time in the foster system, where she befriended another girl named Fish. Shortly after leaving the foster system, Fish died of a drug overdose, leaving Louisa alone again.
Louisa’s time with C. Jat is also short, as police mistake him for an intruder at the auction and arrest him. Not long after, he dies in the hospital of a long illness he’s been dealing with. But before Louisa leaves the auction, she runs into Ted, the man who bought The One of the Sea and who turns out to be an old friend of C. Jat. Louisa hounds the reluctant Ted until eventually he lets her come on a journey to put C. Jat’s ashes to rest. Over the course of this train journey, Ted tells Louisa the story behind The One of the Sea, which primarily takes place over one summer 25 years ago, when the artist later known as C. Jat turned 15.
C. Jat (who, back then, was known as “the artist” or “Kimkim”) had two important people encouraging him when he was younger. One was his friend Joar, who was outgoing and always coming up with schemes. He encouraged Kimkim to paint something for a youth art competition, always with a sense of urgency. Later, Kimkim and others came to realize that this urgency was because Joar was planning to kill his heavy-drinking, abusive father with a knife. C. Jat’s other mentor was Christian, a school janitor who had to leave art school due to addiction. Christian helped C. Jat learn to paint with his authentic self, and the tattoos of skulls on Christian’s arms went on to inspire C. Jat later in life.
As Ted tells the story of that fateful summer to Louisa, it turns out to have several unexpected twists. Joar never had to kill his father because his father was gravely injured in a construction accident. In fact, the person from that summer who ended up dying was Ali, a free-spirited girl like Louisa who moved away and was killed in a surfing accident.
Now, in the present day, Ted takes Louisa to meet Joar, who has been in prison for assaulting a man who was abusing his wife. Now Joar is on house arrest and getting his life back together. Ted invites Christian’s mom, who is now an art history teacher, to come meet Louisa. She is impressed by Louisa’s skill, which reminds Christian’s mom of what she witnessed in C. Jat long ago.
Years after Louisa meets Ted, Joar, and Christian’s mom, she becomes a famous artist and uses her newfound influence to help a new generation of artists. Ted, who was scared away from teaching after being stabbed on the job, finally finds the courage to return to teaching. One day, Louisa calls Ted and encourages him that he should write a book.