Sam (Sean Penn), who works at Starbucks, is an adult, but his intelligence level is only 7 years old. By chance, he took in a homeless woman. The woman gave birth to a daughter for Sam and disappeared without a trace. He named his daughter Lucy (Dakota Fanning) because his favorite song is "Lucy in the Sky with Diamonds" by the Beatles. He loves his children very much. Lucy grew up slowly under Sam's careful care. She found that her father could not understand slightly more complicated stories. She refused to grow up and hoped to be like her father. The life of the father and daughter is so happy. The education officials in the area discovered this special family. They thought that Sam was unable to take care of Lucy, so they took Lucy away. The heartbroken Sam was determined to win back the custody of Lucy, so he self-righteously went to find the best lawyer in the area...