he family includes domineering father Charles (Willem Dafoe), mother Lisa (Julia Roberts), son Michael ( young: Cayden Boyd; adult: Ryan Reynolds ), daughter Ryne (Shannon Lucio), Lisa's much younger sister Jane ( young: Hayden Panettiere; adult: Emily Watson ), Jane's son Christopher (Chase Ellison) and Jane's daughter Leslie (Brooklyn Proulx) and Michael's ex-wife Kelly Hanson (Carrie-Anne Moss).
Charles and Michael have always had, and still have, a strained relationship with each other. For example, as a boy Michael claims he has lost his glasses while he has them in his pocket; even without knowing the latter, Charles, annoyed, has Michael get out of the car and makes him walk home in the rain. Charles imposes a long list of rules on Michael and also on Jane, staying with them. Jane hates these rules too. Michael embarrasses Charles in front of his colleagues by falsely claiming to have written a poem; he is punished by having to keep his arms horizontal while holding heavy objects, such that afterwards his arms hurt so much that he cannot even take food to his mouth; Jane feeds him. One time Charles and Lisa quarrel; in defense of Lisa, Michael attacks Charles such that the latter falls on the ground.
In the "present", Charles and Lisa go to a family reunion at Jane's house, which is Michael's childhood house, to celebrate Lisa's college graduation; Charles drives the car and is in a hurry. They collide with a tree in a manoeuvre to prevent a collision with Christopher, who crosses the road carelessly. Lisa, who did not have her seat-belts on, is killed and Charles is injured. Charles and Christopher both feel guilty.
Between the accident and the funeral Michael cheers up Christopher and Leslie by taking them out fishing, and exploding the fish with firecrackers, like he did with Jane in the past. He encourages the children to lie to their mother Jane about the trip. Charles and Jane consider Michael's behavior inappropriate. Also, after Kelly arrives, Michael has noisy sex with her just after the funeral, with the family and guests in the house hearing it.
Christopher, in an upset mood, runs away. Michael finds him and wants to take him home by car. Christopher insists that he wants to walk home alone. Michael gives him his cell phone in case he changes his mind, but after he leaves Christopher throws it away. He is missing for several hours, and Jane blames Michael for allowing him to walk home. He returns safely.
Michael discovers that Lisa was having an affair with her young professor Addison (Ioan Gruffudd), and had planned to break up with Charles after her graduation. Michael visits Addison; the latter apologizes, Michael says it is OK.
The film title refers to Michael having fun with Christopher and Leslie hitting fireflies in the garden with badminton rackets, in the dark. It is also the title of Michael's book that he had written about his childhood.
After refreshing their memories of happy occasions by watching an old home movie, Charles and Michael reconcile. Michael destroys the manuscript of a book he had written about his childhood, which Charles and Jane were concerned about