Walter is one of the main protagonist of the space adventure game Zathura. He has an older sister Lisa and a younger brother Danny who is the secondary protagonist. He is portrayed by Josh Hutcherson.
At the beginning of the movie, Walter is seen playing a game of catch with his dad. After a few turns, it's Danny's turn to play catch but Walter intercepts the throw causing Danny to storm into the house in anger. Later, Walter is chasing Danny and when they run into their dad's den, they end up ruining a project he was working on leaving him to go and make another copy. While the dad is gone, Walter turns off a video game Danny was playing and starts watching TV. After accidentally throwing a ball into Walter's face, he gets up and chases Danny and finds him in the dumbwaiter. After Danny tells Walter that he's jealous because their dad "thinks I'm smarter than you.", Walter responds by lowering the dumbwaiter into the basement.
Shortly after going back to watch TV, Danny shows up with the board game Zathura. After Danny turns the key and presses the button, a card comes out and Danny has Walter read it to him. After reading the card, "METEOR SHOWER Take Evasive Action", Walter and Danny are surprised when suddenly they see a small meteor come out of nowhere and go through the card. Suddenly, more meteors start falling into the room causing Walter and Danny to avoid being hit. When it's over, Danny looks at a huge hole in the roof. Walter believes that it's just nighttime but Danny doesn't agree. After opening the door and calling Walter over, they both realize they have been sent into space.
When Walter and Danny try to tell their older sister Lisa, she doesn't believe them. As she goes to get ready for her date, she is cryogenically frozen. After reading the instructions, Walter realizes the only way to get back home is to finish the game. After turning the key and pressing the button, Walter gets another card that reads, "YOUR ROBOT IS DEFECTIVE." A robot then appears and while arguing with Danny, the robot grows and starts chasing Walter. After flying out and back into the house, he then stands there waiting to attack Walter. As soon as Danny takes his turn, Walter snags the card and runs from the robot. The card reveals that the house is getting to close TSOURIS-3 causing the house to tilt but eventually the house passes by and Walter sees the robot in the basement. When it suddenly activates and starts to repair itself Walter slams the door shut keeping it trapped. Walter and Danny then take Lisa (who is still frozen) back upstairs and into the bathroom.
Walter then tells Danny that he's going to take his turn but Danny doesn't like the idea. After moving ahead four spaces, Danny refuses to take his turn. Danny goes into the kitchen and wants to make something to eat but, Walter says that because they're in space, there wouldn't be any water or heat (he is wrong on both counts). After getting into another argument with Danny, Danny says that he no longer wants to be treated badly, ignored and treated like a brother. When Walter agrees, Danny takes his turn. The card reads, "YOU ARE VISITED BY ZORGONS". When the Zorgon ship appears it immediately opens fire causing Walter and Danny to try and flee but after narrowly escaping from fire coming down the chimney, Walter takes another spin. When the card comes out it simply says, "REPROGRAM". When Walter holds it up to the Zorgon ship however, nothing happens. Danny is reluctant to take his turn but is eventually forced into it. When the card appears, "RESCUE STRANDED ASTRONAUT", an astronaut appears and has both boys shut off all power. When the astronaut tells Walter to go to the basement and blow out the pilot light, he doesn't because the robot is still in there and simply lowers the thermostat. After getting rid of the Zorgons, Walter tells the astronaut that after he's done eating he wants him out of the house. When the astronaut refuses, Walter holds up a card he previously got that stated he was a fleet admiral. The astronaut then holds up the same one and responds that "it's just a card bugnuts." Since Danny was the one who spun him, Danny gets to choose what happens with the astronaut. Just as the astronaut is about to leave, he brings up the Zorgons and Danny lets him stay angering Walter.
When all three head back to the game, Walter notices that somehow Danny's piece had moved ahead of Walter's, an argument ensues with the astronaut attempting to break it up. Walter grabs Danny's piece and moves it back to the position it was originally in. Shortly after this, a card pops out accusing Walter of cheating, Walter is suddenly lifted skyward and hanging onto the hole in the roof tightly. The astronaut succeeds in saving Walter and bringing him back inside the house. After Danny has a spin, Walter's spin gives him a gold card. Walter says he's gonna win the game. After Danny says he doesn't care who wins, another argument commences with Walter blaming Danny for their parents getting a divorce. Danny, upset runs upstairs. As the shooting star passes, the astronaut tells Walter that whatever he wishes for, nothing good can come of it. After the star passes, Walter refuses to tell the astronaut what he wished for. When the astronaut runs up to the bedroom and finds it empty, Walter appears and says the wish worked. After being berated by the astronaut, Danny appears and Walter says that he wished for a football signed by Brett Favre. When Danny asked why he wished for a football instead of the game ending, Walter says the astronaut yelled at him. The astronaut finally explained himself. He was a player of the game to and was also fighting with his own brother. After receiving a gold card himself, the astronaut wished his brother was never born. After the wish, he felt horrible but, he tried to keep playing in an attempt to get another gold card and wish his brother back but, it wasn't his turn. With his brother having never been born, the astronaut remained trapped in space. Suddenly, the astronaut feels it getting warm and questions if Walter blew out the pilot light. An explosion occurs and when the three of them go to the living room, a huge portion of it is missing, especially where Danny threw the game.
After Danny decides to get the game after the astronaut couldn't, he is grabbed when a Zorgon uses its tounge. He is saved by Walter. As both boys head up the stairs, the robot that had attacked Walter earlier was now fully repaired. As it grabbed Walter by the throat, Walter held up the reprogram card causing the robot to now attack the Zorgons. When Walter spins after Danny, he gets a card that reads, "HIT TIME WARP. Go Back three spaces. Repeat last turn." When Walter's piece goes back three spaces, he is given another gold card. When the star passes, Walter wishes the astronaut had his brother back. Suddenly another Danny appears. When questioned why he wished for another Danny, Walter says he didn't. After thanking Walter, the astronaut places his hand on Danny's shoulder and turns into a younger version of Walter before vanishing. Walter then realises that the astronaut was an older version of himself. As soon as Zathura is reached, it's revealed to be a black hole. With the game ends, Walter and Danny make amends and Walter even asks Lisa if she still thinks he has gorgeous eyes.