Warning: Spoilers for Chemical Hearts are ahead.
The movie, based on the novel Our Chemical Hearts, stars Riverdale's Lili Reinhart as Grace, a transfer student whose boyfriend died in a car accident that left her injured and traumatized. Austin Abrams plays Henry, the boy hopelessly smitten with this strangely enigmatic girl who likes to read Pablo Neruda and dresses in baggy men's clothing.
Once you get to the end, you realize that their love was doomed from the start. Basically from the moment Grace highlighted these lines of Neruda's Love Sonnet XVII: "I love you as certain dark things are to be loved, in secret, between the shadow and the soul." It just took a while for Grace and Henry to accept what was always there.
Henry & Grace's break up, explained
When Henry visits Grace's house one day, he realizes that she's living with her dead boyfriend's family, sleeping in his room, and wearing his clothes. He knows he can't make Grace forget Dom (Jon Lemmon), but he believed her when she said she wanted to move on. It's clear that she hasn't. Still, Henry is prepared to accept all those parts of her (you know, to fulfill that line of the Neruda poem). But when they kiss, he asks why she kisses like she loves him if she doesn't. She says it's the only way she knows how.
And, in that moment, Henry seems to realize that she will never love him back the way that he loves her. He feels led on, used, and hurt, and it's here that he hits his breaking point.
The meaning behind Grace's white dress scene
Even after they break up, it's clear that Henry still cares deeply for Grace. When she doesn't show up to school one day, he races to find her at her secret place: an abandoned, flooded warehouse. He finds her having her own personal Ophelia moment, standing in the water in white dress with a crown of flowers, sobbing.
It's not clear whether she has suffered some sort of trauma-induced break from reality or if she just needs closure so badly that she'll pretend: but she speaks to Henry as if he's Dom. She apologizes for her role in the crash (teasing him to make him laugh while he was driving) and sobs that it should have been her. After she gets out everything she has to say, she does seem a little startled to see Henry standing there and not Dom, but she goes about the rest of her ritual. She sinks all of her photos and letters from Dom into the water, soon joined by her dress and flower crown. She explains that she was going to marry Dom in that dress.
It's clear that Grace does so badly want to move on and move forward. She even previously tried to leave her necklace from Dom at his roadside memorial, but she couldn't help but go back for it. Perhaps she thinks if she ruins her Dom mementos by soaking them in water, that will stop her from going back for them too and start her on her healing process. But as she later explains in her final scene with Henry, "I was never going to be ready until I was ready." No matter the drastic Hamlet-esque steps she tried to take.
Why Grace gives Henry the Neruda poem
After her breakdown, Grace leaves school for a while. And when she returns, Henry avoids her. But on the last day, they run into each other in the hallway and update each other on their lives. He's going to school for writing, she's taking time off for therapy. She kisses him on the cheek (which teenage movie protagonists do 100 percent more times than I've ever seen in real life) and slips a note into his pocket. As she walks away, he pulls the note out to reveal the Neruda poem. She'd lent it to him, and he'd used it as a road map to fall in love with her. Choosing to love her for her flaws and imperfections and understanding that love isn't always based in sunshine and roses.
After they'd broken up, Henry had gone to Dom's grave and set the poem on fire. But the whole thing didn't burn, and Grace must have found the pieces when she went to visit the grave. Instead of being creeped out by that (the movie kind of glosses over the fact that Henry briefly becomes Grace's literal stalker, even following her to the graveyard at one point), she fixes the poem for him. She glues it back together with golden paint like the Japanese art style Kintsugi, which repairs broken pottery with gold so that the object is made whole again but the scars from its break are not ignored.
Earlier in the film, Henry purposefully broke pots to practice the art, but now Grace has used it the way it was meant to be used. To repair something important while acknowledging that in its past it was unintentionally broken. Henry's logic in following the poem was flawed just as the literal poem now has flaws. And, knowing him, he'll choose to love the piece of paper because of those flaws — just like how he loved Grace. But he also knows that he has the poem again because she found it while visiting Dom. So Henry has to accept that Grace won't ever be his, but they'll always share the time they had together — broken as it was.
It's a slightly overwritten but very classic YA ending. Bittersweet and full of metaphor, while both circling back to where it all began, and, with that kiss on the cheek, leaving a little hope for the future.