Read the full summary of the episodes here.
Nine and Twelve are first seen in Episode 1 stealing plutonium from a nuclear fuel reprocessing facility. They leave a spray-painted message at the scene, "VON." VON means 'hope' in Icelandic.
Six months after the incident, Nine and Twelve enroll in a high school in Tokyo as transfer students. While infrequently attending school, they initiate their plan to perform a series of bombings in the city. To foreshadow each attack, they upload a video to the internet under the organization name "Sphinx" in which he and Touji, donned in masks and introducing themselves as Sphinx #1 and #2, leave a riddle that is supposed to lead the police to the site of the bomb. Afterwards, they welcome the police to play along with their game and defuse it. The name "Sphinx" is inspired by the beast of the same name in Greek mythology, who guarded the entrance to the city of Thebes and only granted entrance to those who solved her riddles. Anyone who failed to answer correctly was devoured.
Their first bombing occurs at the Tokyo Metropolitan Government Office. They strategically coordinate their bomb planting with a blackout, giving Twelve 25 minutes to plant bombs (embedded in plush toys) throughout the building while the security cameras are down. Nine, who meets up with Twelve outside the building, learns that Twelve had an unexpected encounter with their classmate, Lisa. Twelve gives him two options: they can either kill her on the spot (and thus avoid the risk of her reporting them) or they can show mercy and save her from the explosion, only to force her to become an "accomplice." Twelve convinces Nine to go with the second, reasoning that ever since the last time they failed to save someone, Nine kept having nightmares.
The second bombing takes place at the Roponggi Police Station. Nine enters the police station dressed as a ramen delivery man, carrying a bomb disguised as an innocent box of noodles. After casually leaving the bomb in a quiet room, he and Twelve upload their second foreboding video. The riddle goes: "What first walks on two legs, then four legs, then three legs?" and it alludes to the Greek myth of Oedipus. The police misdirected themselves to a DNA research facility and were too late to defuse the bomb at Roponggi.
The third bombing targets a shrine, designed as a test to see if the police station has made the connection between Sphinx and the plutonium robbery six months earlier; this reveals that they have indeed made the connection. Nine takes interest in Shibazaki, the detective working for the police, noticing his formidable sleuthing skills but ignoring Shibazaki's publicized warning directed at Sphinx to stop what they were doing.
The fourth attack doesn't involve a physical explosion, but a massive leakage of official investigation reports of Sphinx to the internet. Nine and Twelve designed the riddle in a way that messed with Shibazaki, as his own name was the answer to the riddle. with Around the same time, Twelve brings Lisa to their place, to which Nine expresses disapproval but doesn't force her to leave.
The fifth bomb targeted a subway train, but it did not go as planned. Nine and Twelve plant a cellphone-controlled bomb inside a passenger train, disguised as train staff replacing a fire extinguisher. Afterwards, Nine and Twelve upload another video with the riddle: 'Where did the punishment take place for FEZ 5889, the angel who planted a grapevine?' , noting that the explosion will occur after 8:00 p.m. if the riddle isn't solved before then. This was intended to be one of the easier riddles, and Nine anticipated that Detective Shibazaki would solve it quickly. As the time of detonation approaches, Nine and Twelve are concerned because there were no news updates regarding the bomb being defused. They consider the possibility of the police being unable to solve the riddle. They reveal that it is not their intention to kill anyone, and attempt to deactivate the bomb using their cellphone. Strangely, it so happens that cellphone reception has gone out all over the area, preventing them from doing so. At this point, they suspect that someone is onto them, and are left with no choice but to set out and defuse the bomb manually. While trying to pinpoint the train's location on his computer, Nine is horrified when he sees that his own computer has been hacked into, and suspects that the hacker must be Five. He manages to find the train's location using a second computer, and realizing that it is currently stationed close to his hideout, rushes over and drops a smoke bomb to force panicking commuters off the train car and cover his tracks.
However, one of the passengers is oblivious to the entire situation as she is asleep, and Nine prioritizes saving her first. The bomb goes off before he has the chance. Nine manages to pull himself and the commuter out of the wreckage with minor injuries, right afterwards receiving a text message from Five that reads, "I found you ^ ^."
After the train incident Nine has a nightmare involving Five, he and Twelve discuss about her, and Nine concludes that Five is continuing a "game" the three played together previously. When Lisa offers to help
them, Nine shows her pictures of the aftermath of the subway train bombing and states that this is what they were doing, which shocks her. Later on Five sends a text containing a riddle to the public: 'GIGFHEEGMHJL FD. Gaius Julius Ceasar gone to Arab. Sphinx.', posing as Sphinx; the riddle can only be decoded via Caesar cipher. The answer turns out to be a set of coordinates leading to Haneda Airport's International Terminal, one of the locations targeted by Nine and Twelve. Knowing that they will be caught by Five while trying to enter the terminal, Nine reluctantly decides to bring Lisa along with them, since she isn't known to be associated with him and Twelve. Upon arriving at the terminal, Nine realizes that Five plans to play a game of chess using the airport as the chessboard.
Nine and Twelve decide to play along with Five's chess game. When they get closer to the bomb's potential location, Twelve sends in Lisa to create a distraction; after some hesitation, she sets off flares that trigger a fire alarm. Twelve then hacks into the camera system and brings the recordings back by five minutes, allowing Nine to rush to Five's location without being detected. He confronts Five and holds her at gunpoint with a pistol he brought, demanding to know why she is getting in the way of his and Twelve's plans. She responds that she wants to settle something with him, then reveals her knowledge of Lisa being an accomplice and that she is being held captive at the bomb's location. Airport police then arrive and fire at Nine, forcing him away. While fleeing, he briefly passes Shibazaki. Nine regroups with Twelve, who receives a phone call from Lisa describing her and the bomb's whereabouts: a plane that is on autopilot and headed for a terminal filled with numerous
commuters. Realizing there is a way to save both Lisa and the commuters, Nine contacts Shibazaki, convinces him that the latest bomb isn't Sphinx's, and instructs him to infiltrate the control tower and take control of the autopilot system. Nine and Twelve rescue Lisa from the plane. The plane explodes without injuring anyone, and Shibazaki spots a masked Nine, who signals him in thanks before leaving.