Old Man is at his weekly Canasta game with some other old people, when an unexpected telegram arrives for him. He never gets any mail except from Fat Kid, and Fat Kid always uses a special envelope so Old Man knows it's not from him. Old Man puts on rubber gloves to open it, and carefully removes the letter from the envelope. Just as he suspected, it is a ransom note from a terrorist. Fat Kid has been Fat Kidnapped. The note says to fly 10,000 rubies to the top of the Eiffel Tower, or else they're going to throw Fat Kid to his death. These terrorists aren't fucking around. Old Man is so shocked by this terrorism that he has a heart attack. One Week Later... Old Man wakes up in the hospital. Some detectives are standing around him. They need to ask him a few questions, because Fat Kid has been missing for a week. Old Man is at the top of their list of suspects. Old Man is like "How could I have done anything, I've been in a coma for a week" and the police are like "Oh yeah. Never mind." And then they leave, embarrassed that they wasted a week waiting for this old guy to wake up.
After the police leave, Old Man remembers the ransom note from earlier. He realizes that there's no time to tell the police - he needs to get the top of the Eiffel Tower as soon as ASAP. He finds a scalpel and starts cutting into his leg. After he cuts it open we see that he has an emergency supply of balloons in there. He blows up all the balloons and ties them to the hospital, which then floats UP into the air and begins traveling to Paris. Hundreds of people die in the hospital but nobody can stop Old Man because he locked the door to his room. Later, in Paris, Old Man makes it to the Eiffel Tower and tries to land the hospital on top of it, but the tower is pointy so it cuts the building in half, killing everyone inside except for Old Man because he jumps out at the last second and lands on a platform at the top of the tower.
Old Man starts running around the tower, searching for Fat Kid and/or the terrorists. A French mime blocks Old Man's path and starts pulling an invisible rope, but Old Man doesn't have time for this bullshit so he beats the mime to death with his cane. But then he realizes that the mime was just trying to warn him, because there is a terrorist holding a rope standing right next to him, and he ties up Old Man and drags him into a trap door and then to a secret room inside the Eiffel Tower. Fat Kid is in there too, he's tied up with a bunch of terrorists standing around. The leader of the terrorists is like, "Thank you for coming, my old friend..." and he pulls off his mask to reveal that he's Doug the dog from the first movie. Old Man is like "Why did you do it, Doug? I thought you were cool." Doug is like "Shut up-a your face", when suddenly Fat Kid manages to eat through the duct tape covering his mouth. Fat Kid's giant tongue reaches out of his mouth, all the way across the room and licks the back of Doug's head. Some sparks fly out of Doug, and he falls to the floor in pain. A small computer chip falls off head, and Old Man realizes that this is what was controlling Doug the whole time. Fat Kid is like, "See, he's still cool. Good thing I short-circuited the chip with my giant tongue."
Later, Old Man and Fat Kid have escaped. They tied some balloons to the dead mime and are riding his corpse back home. Doug decided to stay in Paris, he couldn't face Old Man and Fat Kid after all the evil deeds he was forced to commit. Fat Kid is like "Thanks for saving me, Old Man, it ended up being easier than I expected!" Old Man replies, "Yes... a little TOO easy." Old Man turns up his hearing aid to the highest possible setting, and hears a ticking sound coming from inside Fat Kid's stomach. He then turns his glasses up to the highest X-Ray setting and sees that Fat Kid has a ticking time bomb inside him, and there's only five minutes left until it explodes. Old Man is faced with a difficult choice: Stay with Fat Kid and support him until he explodes, killing them both, or push Fat Kid off the floating mime so that Old Man can live to die another day. Fat Kid is like "Hey Mr. Old Man, I bought you this friendship bracelet while I was in France!" As soon as Fat Kid gives him the bracelet, Old Man pushes him off the floating mime to his death.
As Fat Kid falls through the air, and Old Man floats away on a French performance artist whom he earlier murdered, they each look back on their lives, thinking about any regrets they may have. Old Man flashes back to when he was a young Old Man, in Bomb Squad Academy. His drill sergeant is yelling at him, "Listen up, Old Man! If you don't learn to defuse this bomb, then someday somebody you care about is going to explode! Is that what you want?" Meanwhile, Fat Kid flashes back to his time in Fat Camp. His drill sergeant is yelling at him, "Listen up, Fat Kid! If you don't stop eating, then someday you're gonna eat something that's going to end your fat life! Something like a French time bomb, for example! Is that what you want? You want to eat a time bomb?" Fat Kid keeps falling, understanding that this is all his fault. Old Man keeps floating away, understanding that this is all Fat Kid's fault. Meanwhile, in Paris, Doug the dog can't live with the guilt of everything he's done. He puts a gun in his mouth and pulls the trigger. Old Man returns to his retirement home and continues his Canasta game.