<P> David encourages Dennis to be from Mars only at home; he has to be from Earth everywhere else . Dennis passes his interview by saying he was pretending and continues in David's care . Later, David, now Dennis's adopted father, tries to insist that Dennis acknowledge being from Earth, to which Dennis responds with hurt and anger . David leaves Dennis with Liz to attend the reveal of his new book, which is supposed to be the sequel to his first book . David confesses to Tina, the publisher, that he has not written a sequel, but rather a new book titled, Martian Child, about Dennis . In her fury, Tina makes a scene at the party, but takes the manuscript as David leaves to be with Dennis . </P> <P> Meanwhile, Dennis has walked away from Liz's house, together with his suitcase of earthly artifacts . When David arrives and finds the police at Liz's house and learns the boy is gone, he remembers the place Dennis had identified as where he was found . David asks Harlee to drive him to the location, where they spot Dennis high up on the outside ledge of the museum's domed roof . David climbs up to where Dennis is as the police and Liz arrive . Dennis identifies a bright searchlight in a nearby cloud as someone coming to take him home, but David tries to assure him that it's just a helicopter . David pledges his love for Dennis and assures him that he will never ever leave him . Eventually Dennis gives in to trusting David and the two embrace . </P> <P> David's voiceover tells about the parallel of children who come into our world, struggling to understand it, being like little aliens . As we zoom in on Tina reading the manuscript aboard an airplane, we realize these are words from the book . Tina is crying . </P> <Ul> <Li> John Cusack as David Gordon - Author and Dad <Ul> <Li> Zak Ludwig as young David </Li> </Ul> </Li> <Li> Bobby Coleman as Dennis - "Martian" and Son </Li> <Li> Amanda Peet as Harlee - Friend </Li> <Li> Sophie Okonedo as Sophie - Foster Mom </Li> <Li> Oliver Platt as Jeff - Agent </Li> <Li> Joan Cusack as Liz Gordon - Sister and Aunt </Li> <Li> Anjelica Huston as Tina - Publisher </Li> <Li> Richard Schiff as Lefkowitz - Child Services Authority </Li> <Li> Howard Hesseman as Dr. Berg </Li> <Li> David Kaye as Andy </Li> </Ul>

What happens at the end of martian child
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