NOTE: This spoiler was submitted by Jeremy

The film starts with Peter Decker (Oakes Fegley) entering middle school with his best friends Billy (Juliocesar Chavez), Steve (Isaac Kragten), and Emma (TJ McGibbon). They are all unhappy about being the youngest kids in a new school, with Steve worried about his older sister Lisa (Lydia Styslinger) making fun of him and Peter getting picked on by an eighth grade bully (Drew Scheid). To top things off for Peter, he is most unhappy that his grandfather Ed (Robert De Niro) is moving in to take his room.

We see Ed from not long ago in a grocery store being rude to the manager (Faizon Love). When Ed didn’t understand the self-checkout system, he just took the groceries with him without paying. The manager chased after him, but Ed thought he was getting harassed and ends up falling down. This caused a group of people to think the manager was the one trying to hurt Ed, and everyone pelted him with garbage. Ed’s daughter Sally (Uma Thurman) visits Ed in his home, saying that it would be better that he moves in with her family since she knows that he is still mourning the loss of his wife. Although Ed is reluctant, he eventually comes along.

When Peter finds out that he has to stay in the attic now, he argues with his dad Arthur (Rob Riggle), but he and Sally counter that Peter’s sisters Mia (Laura Marano) and Jennifer (Poppy Gagnon) share their room and get along fine (they don’t). Ed soon arrives, and while his grandchildren show him love, Peter is not the least bit eager to give up his room. He relents and goes into the attic, which is inhabited by a bat and a rat that make it hard for him to sleep or get homework done, and the roof leaks right over his bed. Peter then slips a note under Ed’s door to declare that if he does not give Peter his room back within 24 hours, he will declare war. Ed just brushes it off as a playful child’s thing.

Sally brings Ed to spend time with his old friend Jerry (Christopher Walken), who lives by himself in his own man cave. They are joined by another friend, Danny (Cheech Marin), and Ed shows them Peter’s note. They think he ought to take it seriously and with back, but Ed doesn’t think Peter will really act on it. He is proven wrong later that night when Peter uses a remote controlled car to play loud music on his phone to wake Ed up and annoy him. Ed goes to talk to Peter over this, but the boy remains firm in his demands and doesn’t plan to back down.

The next day, Jennifer goes into Ed’s room to ask for one of his marbles. When he tries to open the jar, Ed finds that Peter glued it down to the table, and it breaks when he tries to pull it off, sending all the marbles rolling all over the place and causing Ed to slip and fall. Ed later calls Peter down to come to terms with their war. They agree to not let the rest of the family get caught in the middle of things.

Peter plays more pranks, such as rigging Ed’s record player to send the record flying, or replacing his shaving cream with foam sealant that hardens around his face. He also puts hot sauce in Ed’s coffee, except it was Sally who took it and drank it, then ended up throwing it out her window just as a cop drove by. Ed retaliates by changing Peter’s summer writing essay to say silly things that he doesn’t notice until he has to read it out loud in class, and then remove the screws from Peter’s desk and bed while he’s at school. Meanwhile, Sally comes home to find Mia with her boyfriend Russell (Colin Ford), whom Sally is not fond of at all. Peter and Ed continue the war with Peter sabotaging Ed’s golf club and Ed bedazzling Peter’s gym sneakers.

Ed goes to a grocery store where he meets cashier Diane (Jane Seymour), who finds his purchases weird, until he says outright that he is in a war with Peter. Ed appears to hit it off with Diane as well. Back at home, he tries to give Sally advice on dealing with Mia having a boyfriend, as Ed did when she was dating Arthur, but he also uses it as an opportunity to belittle Arthur’s career.

The war continues with things like Peter planting a snake in Ed’s bed, or Ed destroying Peter’s castle in his online game. Jennifer tries to mediate a truce between them, but Peter is too stubborn to agree to it. The family later goes to Sky Zone to play, with Peter and his friends competing against Ed and his friends, including Diane. Both teams play a tough game, but when Ed and Peter are the two remaining players, they tie the game and and argue over who won until they are forced to leave.

Ed later picks Peter up from school in an earnest attempt to bond with him. He takes him to a lake to go fishing, and they manage to catch some fish until a man in a canoe tells them fishing is illegal there, and a ranger is heading toward them. Although the boat slowly gets them back to the dock, Ed and Peter manage to evade the ranger. Ed later takes Peter by his old house that he now has to sell, telling Peter that he helped design and build it. Despite having a good time, Peter cannot help but prank Ed again by loosening the hinges on his door.

Ed pranks Peter by putting an exploding creamsicle in his bag. The 8th grade bully opens Peter’s bag to dump food in there, only for the creamsicle to explode in his face. Thinking it was Peter who did it, he gives the boy a bloody nose. Peter gets Ed back by making his cellphone go off during a funeral, leading to Ed dropping the phone into the dead man’s pants somehow.

Ed and Peter put aside their bickering as Jennifer’s birthday is coming up and they agree to behave for her sake. Ed even gets Jerry and Danny to get back at the bully by throwing him in a dumpster. The family and others come together for Jennifer’s party, which has a Christmas theme to it. However, Peter set up a prank by putting an airbag under the Santa seat, thinking Ed was gonna play the part of Santa, but it turns out Jerry is filling that role. The rat from the attic then chews on some lights, which Ed grabs just before Peter plugs them in. This causes Ed to get a slight electrocution and have the ladder fall onto the bouncy house. The airbag goes off and sends Jerry flying. He gets up all dizzy and knocks over the Christmas tree, which falls onto Arthur’s grill and catches fire. In the chaos, Sally falls face first onto the cake. Then the sleigh decoration slides off the roof and hits another tree, which falls against the house and smashes into the girls’ room, where Mia and Russell are hooking up again. Ed and Peter realize they have gone too far since Jennifer is sad at her party being ruined.

The family goes to the hospital for Ed’s injuries. Russell goes to comfort Mia, but Sally sees him and chases him out of the hospital. She ends up tackling him, but after talking it over with him, she decides to accept Russell as Mia’s boyfriend since she sees he is harmless and actually cares for her. When it’s time to take Ed home, the nurse tells the family that he left already. They go back to the house and see that Ed took his things and left, which makes Peter feel horrible over starting the whole war. He then realizes he knows where Ed might be. He has the family drive to his old house where an open house is taking place. Peter finds Ed and apologizes to him, saying he will let him keep his old room and that he cares about him. Ed embraces his grandson.

Later on, Ed moves back in with the family. He and Peter have spent time fishing, but now Ed is going on a date with Diane. Peter looks out the window to see them, but with a jealous look in his eye. Ed tells Diane they had better go now.

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Peter Decker has to give up his bedroom for his grandfather Ed after Peter's mom Sally thinks that her dad needs to be with family since he lives alone and is still mourning his wife's death. Peter initiates a prank war against Ed so that he may give him back the room, but Ed is not going down so easily.

Although Ed tries to genuinely bond with Peter, the boy won't back down, so Ed pulls some pranks of his own. This ends badly when they ruin the birthday party of Peter's little sister Jennifer, which also causes Ed to get injured. Ed gets his things and plans to move out, but Peter finds him and apologizes for how he behaved and will give up his room for him. Ed forgives him and moves back.