On February 15, 1995, Katagiri, a longtime collections officer for the Tokyo Security Trust Bank, is surprised to find a giant frog waiting for him politely outside his apartment. Frog introduces himself and apologizes for barging into Katagiri’s home, claiming to have an urgent matter to discuss. He has come because Tokyo is about to be destroyed. Katagiri thinks that it must be a prank, but Frog assures him that they are the only ones present. Frog tells him that he isn’t crazy and that Katagiri isn’t dreaming. Confused and overwhelmed, Katagiri asks whether Frog is “a real frog.” Frog replies in the affirmative and demonstrates his realness by tilting back his head and letting out a wall-rattling series of ribits. Frog tells Katagiri that there will be an earthquake three days from now, much bigger than the one that struck Kobe a month earlier. More than 150,000 people will die, and the city’s infrastructure will be destroyed. The epicenter will be right under Katagiri’s bank office. Frog tells Katagiri that to stop the earthquake from happening, the two of them must go underground and do mortal combat with Worm.
Frog explains that Worm is a giant worm that has slept deep beneath Tokyo for decades, his eyes and brain slowly atrophying. Worm absorbs reverberations as he sleeps and replaces those impulses with rage through a mysterious chemical process. When the rage builds to a certain level, he releases it in the form of an earthquake. Frog says that he feels no personal animosity toward Worm but that Worm has nevertheless become too dangerous to ignore. Katagiri accepts this explanation, but he still doesn’t understand why Frog has chosen him for the mission.
Frog says that he’s been watching Katagiri for a long time. He’s seen Katagiri do the dirty, difficult work of collecting money from thugs without ever receiving the recognition he deserves. He’s also seen the sacrifices Katagiri made for his siblings. Frog says there is no one in Tokyo he would rather have by his side in this battle. Katagiri suggests that Frog will be better off with someone who’s stronger, maybe a martial arts expert. Frog waves him off, saying that he’ll do the fighting but needs Katagiri for moral support. Frog is also frightened and reluctant to fight Worm, but he quotes Nietzsche’s maxim that “the highest wisdom is to have no fear.”
As a final reassurance, Frog offers to solve a sticky loan situation for Katagiri involving a company called Big Bear Trading. Then Frog flattens himself up and slips out through the door crack. The next morning, a lawyer calls Katagiri to say that that Big Bear will pay off all debts as long as Frog stays away. Frog comes to Katagiri’s office at lunch. He tells Katagiri not to worry because no one else can see him. Now that he’s proven his existence, Frog wants confirmation that Katagiri will help him fight Worm. There will be no glory in their battle, he warns, because no one besides the two of them will ever know it has taken place. Katagiri tries to convince Frog once again that he’s not worthy, but Frog says that people like Katagiri are the reason why Tokyo is worth saving and that only someone like Katagiri could help him win the battle. Katagiri reluctantly agrees.
Frog describes how the two of them will head underground through a hidden shaft in the bank’s boiler room on February 17. He assures Katagiri that he has battle plans but refuses to divulge them when Katagiri asks for details. Katagiri pushes again, asking what Frog will do if Katagiri gets scared and runs away. Frog thinks and then answers that he would just have to fight alone, even though he would have about as much hope of defeating Worm alone as Anna Karenina would have had of beating the speeding locomotive that killed her. Frog seems disappointed that Katagiri has not read Tolstoy’s novel.
On the evening of the battle, however, Katagiri is shot while returning from his collection rounds. A young man in a leather jacket jumps in front of him and puts a slug in his right shoulder. Other shots follow. Katagiri wakes up in a hospital, where the attending nurse explains that it’s 9:15 a.m., February 18. Katagiri asks about the previous night’s earthquake and is relieved to discover that nothing happened. He’s perplexed, however, to learn that he wasn’t shot. He was found lying unconscious on the street and had apparently been yelling Frog’s name in the hospital all night long.
Frog appears in Katagiri’s hospital room that night, exhausted and wounded. Katagiri begins to apologize for not meeting him in the boiler room, but Frog explains that Katagiri did help him—in his dreams. The battle took place in the area of imagination, Frog explains. He describes how Katagiri brought in a foot-powered generator to light Frog’s way in the battle to fight off the encroaching darkness. In the end, Frog was unable to defeat Worm, able only to call a draw and prevent the earthquake from taking place.
Frog then slips into a coma. He begins to twitch, and boils break out all over his body. The boils pop, and creatures that look like worms, maggots, and centipedes start crawling out. Frog’s eyeballs fall to the floor, where they’re eaten by bugs. The entire room is covered with insects, blotting out the light, overwhelming Katagiri in his bed. He screams. The nurse turns on the light, and the bugs are suddenly gone. She gives Katagiri an injection and tells him that he had another nightmare. Delirious, Katagiri tells the nurse that Frog sacrificed himself to save Tokyo from an earthquake. The nurse says that Katagiri must have been very fond of Frog. “Locomotive,” he says. “More than anybody.” She wipes the sweat from his brow as he drifts into a deep sleep.