Nathan looks at pictures of his three kids playing in the ocean water from a past vacation. On their last night at the beach, they ate at a seafood buffet and his daughter, Bea, went to bed early because her stomach was hurting. The next morning they left the hotel early and everything was fine. 

Marcy, Nathan’s wife, tells him that everything is his fault. When they left the hotel, Nathan remembers driving and thinking Bea was asleep in the very back seat of the van. Instead, he discovered after seven hours of driving that she had disappeared.

Every July, the family goes to Destin and usually spends time at Destiny Cove. That year, Bea requests to go to Cocoa Beach, so they book a reservation at the Beachcomber Inn. His son, Andrew, throws shells at seagulls and the oldest daughter, Cat, texts boys on her phone. Nathan remembers falling asleep on the beach with a headache and waking up to Marcy screaming Bea’s name. The other two children explain that they were all playing together in the dunes when Andrew found a thick green bottle with the name “Golden Sun,” the name of Bea’s song, inscribed in the sand. Bea tried to drink the bottle but Andrew broke it with a rock. Marcy consoles the children and is concerned when she finds Bea’s eyes glazed over instead of crying like she usually does. Nathan remembers telling Cat to get her sister and brother into the van and they leave around 9 a.m. A few miles before they exit Florida, Nathan stops for gas and glances at a blanket in the back seat he assumes Bea is lying under. As the family sleeps for the next few hours, Nathan drives with a headache and hums Bea’s song, Golden Sun. He is disturbed by the fact that the song’s name was found on the bottle. His headache began after Bea chanted the song a few days prior and he is unable to shake the song or the pain. 

Cat tells Nathan how the night before their last night at the beach an old man approached them on the beach and handed them slices of bread to feed the seabirds. As Nathan reflects on the mystery of his daughter’s disappearance, he recalls discovering later online that there is no drink called Golden Sun. He feels as though Cat and Andrew are still not telling him something. In the way back of the car underneath the blanket he’d mistaken for his daughter, Nathan finds a Golden Sun bottle cap. 

On some mornings, Nathan stands in his driveway and stares out at the back window of the van and he can almost see Bea in the window’s reflection. 

Marcy dreams of how Bea felt the last day they spent together. She remembers Bea chanting “Golden sun gonna come for me, golden sun gonna come for you” so often that everyone began to join in. Like her husband, Marcy tries to remember the details of the days leading up to Bea’s disappearance. At night, Marcy recalls how Bea whispered the chant and suddenly she remembers that a girl in her seventh grade class sang the same song before she disappeared. 

She mouths the words to the song and feels dizzy. She thinks she saw four blurred outlines of people in the distance at the beach, instead of just the three of her kids, but she brushes it away at the time. When her daughter approached her crying, Marcy remembers thinking that although her appearance hadn't changed, the child in front of her was not Bea. 

Nathan thinks that Marcy blames him for the disappearance, but really she is too nervous to tell him about the girl she knew when she was young who suffered the same fate. When she’s alone, Marcy sings Bea’s song to herself and hopes it will make Bea come back. 

Cat recalls incorrectly telling her parents that the man on the beach who handed them bread was old. In fact, it had been hard for her to determine the age of the person, whose appearance seemed to shift with the light on the beach. The man told them to tell their parents he gave them bread for the seagulls, but Cat can’t remember if this actually happened. When he sang Bea’s song to them, the clouds above grew dark. Cat remembers trying to divert the man’s attention to protect her sister, but it didn't work — he wanted Bea all along.

She has a false memory of the man waving to their parents and Marcy and Nathan waving back before the man takes them further down the beach, a memory the parents don’t share. Cat recalls how the man tells them, singing, how if they find the sun they will shine forever. Afterwards, Cat tries to ignore the memory and Bea’s singing. But, she can't ignore Bea when they go back to the beach and she digs in the sand, suddenly stopping to say that the sun is not the man’s to find. That’s when the children find the Golden Sun bottle and all feel compelled to drink from it. Cat controls herself, but Bea can't and Andrew knocks the bottle out of her hand.

At the seafood restaurant on their last night, Andrew leans over to Cat, crying about how the memory of the bread isn't real. On the van ride back, Cat gets the feeling that the Bea in the car is not the genuine Bea. Her dad questions her constantly, but she doesn’t know anything for sure. The one thing she knows is true is a drawing she does of a circle with a starburst inside of it. 

Andrew remembers the additional verses of the song that Bea didn’t sing. He recalls the man singing “the sun gonna burn as it sets you free, sun for you, and sun for me.” He remembers even more, but is too scared to repeat it. Sometimes, he goes into his father’s office and holds the bottle cap and cries. Over time, he begins to question Bea’s existence. He remembers finding the bottle and after Bea drank from it, it turned into the man who appeared with a halo. 

In the night, Andrew wakes up and goes to the window and stares at the bottle cap in his palm and tries to remember what the man said to them on the beach. He remembers a promised future where he lived next to the ocean with a group of boys and girls, running wild with no parental restrictions. But then, Andrew remembers seeing a leviathan emerge from the dark water and as he does so the bottle cap trembles in his palm and he spits up muddy water filled with swimming creatures onto his bedroom floor. He closes his eyes and the bottle cap bites his palm which he then places on the window. When he opens his eyes, he sees Bea on the other side of the glass, standing in sunshine as they sing together.