A science fiction writer adopts a boy with socialization problems who claims he is from mars. John Cusack plays the lead character David Gordon who is a science fiction fantasy writer. He sees his female friend Harlee (played by Amanda Peet) at the gravesite of his wife Mary who passed two years prior. They talk about David adopting a child. David is considering it, but is unsure if he would make a good parent as a single widower. David goes to see Sophie (played by Sophie Okonedo) who runs an orphanage. He changes his mind about adoption and tells Sophie he is not ready. He learns about a boy named Dennis (played by Bobby Coleman) who is very unusual. Dennis stays each day inside a cardboard box because he does not like the sun.
David goes to visit his sister Liz (played by Joan Cusack) who warns him parenting is really difficult. But David continues to think about it, since it was his wife's wish to adopt a child, since she herself was adopted. David is very lonely, missing his wife and living alone with only a dog named “Somewhere.” David goes back to the orphanage. He gives Dennis some sun block. He learns Dennis believes he is from Mars. Dennis has a hobby of taking photos of everything and everyone because he says he is learning to be human. Dennis waits outside in his box in the rain for David to return. David does come back but the next day. David gives Dennis sunglasses. Dennis thanks him. David plays ball with him with the box turned on its side. Slowly Dennis emerges from the box. They start to become friends. David decides to adopt him. David goes before the board, who must decide if they approve the adoption. David waits anxiously by the phone for the call from them. He is approved. He meets with Dennis and asks him if he wants to come live with him. Dennis says he eats Lucky Charms cereal. When he gets to David's house the dog knocks him over. Dennis is frightened but not hurt. David has a night light, which puts the solar system on the ceiling. Dennis tells him the scale is wrong. He tells David he knows the Earth should be shown much smaller. He also says it took a very long time for him to get to Earth. David makes him a grilled cheese sandwich, but Dennis refuses to eat it. Dennis will only eat Lucky Charms. They go to the store and buy a shopping cart full of Lucky Charms cereal. Late at night Dennis is at the foot of David's bed watching him. David awakes and Dennis takes his photograph to add to his collection.
Mr. Lefkowitz (played by Richard Schiff) calls to check on Dennis. Dennis has incredible curiosity about everything. David takes him to a baseball game. David says his team needs a hit. The next batter gets a hit. Dennis says he made a Martian wish. David asks him to wish the next batter gets a home run. Dennis says OK. The player hits a home run. Dennis asks if David wants them to win the game. On the way home David says the game was great. They get stuck in traffic and David says he wants the lights to change. Suddenly they change to green. Dennis claims to have martian wish power. Dennis asks David if he wants a Martian wish. David says yes. Dennis gives him one. The next day David and Harlee teach Dennis to play baseball.
Jeff (played by Oliver Platt) comes over to see David. Jeff is his literary agent. Jeff tells him his publisher Tina (played by Anjelica Huston) is coming in six weeks and expects him to have the draft of his book finished.
Dennis has his first day of school and gets picked on by the other kids for hanging upside down in the playground. Dennis steals from the other children. This gets him expelled from school. David takes the boy to a psychologist Dr. Berg (played by Howard Hesseman). Dennis wears a weight belt because he thinks he will float away. Dennis measures things with a tape measure. Dennis remembers he was abandoned in the park by the telescope dome and left in the rain. Dennis tells him he can taste color. They use M&Ms as a test. Dennis covers his eyes and gets them all correct, except the blue one. He says he cannot taste blue. Dennis renames the dog “Flomar” which Dennis says is Martian for warm furry friend. Dennis says the dog chose the new name. David starts to think Dennis story about being from Mars may be real. David is having trouble adjusting. David is also having trouble writing. He goes to Liz's for Thanksgiving and Liz upsets him by telling David to give Dennis back.
In David's home, Dennis breaks something by accident. Dennis get worried he will be sent away. David tells him he will never send him away no matter what he does. David breaks something on purpose. He encourages Dennis to break more stuff. They smash up plates and make a mess of the house having a really good time. At this moment, . Lefkowitz frmo Child Services shows up and harshly judges the scene. Mr. Lefkowitz tells David the adoption case is up for review, then leaves. David asks Dennis why he took his photos of his wife. Dennis says it is because David loves her so much. David tells Dennis they must please Mr. Lefkowitz because he wants to take Dennis away. Harlee and David have a nice talk. They flirt. They kiss.
Dennis goes to a new school not wearing his weight belt. David stays where he can see him in the car and waits all day. When they come home Dennis teaches David to dance in the Martian way. After they dance Dennis tells him they had a nice talk. The dog dies. David cries. Dennis comforts him. They go look at stars together. Dennis points out Mars. Dennis says his mission is almost over and he will return to Mars soon. David takes Dennis in for an interview with Child Service. Dennis impresses them very much with his improved social skills. Dennis admits he was only pretending he was from Mars because he did not fit in. Child Services congratulates David on the improvement. Dennis does lots of regular things like other kids. They go to a science museum and see a display about Mars. Dennis does not like it and says it is not like he remembers. They have an argument in the car going home. Dennis once again tells David he is from Mars. David gets upset, saying he thought they were over this. David hits a police car. His license is missing. David gets mad at Dennis and accuses him of stealing it. David has Liz baby-sit Dennis so he can go out.
David goes to the book signing. He does not have the book he was supposed to write, instead he gives Tina a book about Dennis called “Martial Child.” Tina gets furious. David comes home to find Dennis is missing. Dennis has made a wonderful contraption with the photos of him and David that are projected with a light onto the ceiling. Dennis goes to the telescope dome where he told David he was first found. Dennis climbs out on the roof. David goes after him knowing where he will go. Dennis tells David his mission is over. He wants to go back to Mars. Dennis sees a helicopter and says they are coming to take him home. Dennis says he is sorry he must leave. Dennis says he will tell them about baseball and all the fun things they did. David tells him he wants to prove to him he belongs to him. Dennis asks David why did his parents leave him. David says because they were stupid. David convinces Dennis he will never leave him. Dennis goes to him and David says he got his one Martian wish. David brings him home safely. Tina reads the book about David's experience with Dennis and cries because the book is so touching. Dennis and David have a backyard party where everyone looks very happy.
Best part of story, including ending:
The way the story unfolds, it makes one wonder maybe this kid really is from Mars, which is very interesting.
Best scene in story:
I like the scene where Dennis says he can taste colors and proves it with the M&Ms by guessing all the colors correctly, except blue. When Dennis says he cannot taste blue it is a wonderful moment, because the details are so convincing.
Opinion about the main character:
David is amazing in his acceptance of the little boy's quirkiness because it reminds him of the way he was when he was a child. Only a person who knows how it feels, to be the odd one, would have the depth of understanding to help the boy. David's patience is phenomenal.