In Death Note, the death of L, the world's greatest detective, was just a temporary victory for Light Yagami. Five years later, a ruthless rogue calling himself Mello appeared to take Kira down, and with a wild scheme in mind.
Already, Mello was competing intensely with the boy genius Near to find Kira and obtain the Death Note, and Mello's reckless determination to win drove him to kidnap an innocent to gain the Death Note. Light was not prepared for this.
Mello's plan to obtain the notebook came out of nowhere, and Light had absolutely no defense against it at first. He'd grown complacent over the five years since L's death. He had "proven" his and Misa's innocence earlier and now worked even more closely with the police, but suddenly Light's position ad become fragile. His own family was at stake, and not even he could just give up on his younger sister, Sayu Yagami.
Working with an American crime family, Mello captured the director of the Japanese NPA, but Light killed the director to prevent any deal from taking place. Undeterred, and half-expecting that to happen, Mello moved on to a more personal target: the daughter of vice-director Soichiro Yagami. One night, Mello's men captured Sayu on the streets as she walked alone, and she was flown to the southwest United States and held in an underground facility. Mello then made some cryptic phone calls to the NPA, speaking directly to Soichiro. Mello knew that the Japanese police and L had a Death Note, and the upright Soichiro Yagami wouldn't sacrifice his own daughter to protect it. Mello sent evidence that Sayu was alive and well, and demanded that Soichiro Yagami fly to Los Angeles to prepare for a trade. Sayu would be returned unharmed, but only if Mello got his hands on the NPA's Death Note. Light struggled to find a way out of this, and not even the surprise assistance of Near, a very different kind of Kira hunter, could do much good. L and Near were running out of time, with both Sayu and the Death Note at stake. When the time came, Soichiro found himself at the airport, unable to escape Mello's machinations.
Mello had an agent at the airport, ready to meet Soichiro: a certain Zakk Irius. Mr. Irius gave Soichiro a tiny headset that connected him to Mello, and through Soichiro, Mello demanded that L prevent any news coverage of what was about to happen. Indeed, the entire world was ignorant when the plane's pilot (who was on Mello's side) redirected the Los Angeles-bound plane to a seemingly empty stretch of the American Southwest's wilderness. Soichiro disembarked as ordered, and happened upon a hidden underground facility. It was time to trade.
As promised, Sayu was there, contained behind the thick glass of a revolving set of doors. Another of Mello's agents was there, and Soichiro offered the notebook through a hole in the glass to allow them to test it. The notebook was real, and through his agent Mello killed one of his disloyal mafia minions to verify the its power. Thus, Soichiro passed on the notebook, and the revolving doors turned enough to pass Sayu on to her father. Soichiro hurried back to a waiting helicopter that would take them to Los Angeles, and elsewhere, Mello's agent retreated in a different helicopter. Light and Near both watched helplessly, waiting to see what would happen. And Ryuk wasn't about to help Light, either; as usual, he was only an observer.
They were in for a surprise: the agent's helicopter exploded, and a missile was launched soon after. The notebook had been loaded into that missile, headed to who knows where. Worse yet, the missile was impossible to track, meaning L and Near couldn't account for the notebook's ultimate fate. Unbeknownst to them, the notebook's missile ended up near New York, and Mello's agents there retrieved it in the Hudson Bay. The notebook was then covertly sent to Mello's HQ in Los Angeles, and with it, Mello killed off most of his untrustworthy fellows and Near's SPK agents. Now he was the dominant player in the battle for the notebooks, especially since he now knew Soichiro and Sayu's names and faces. In theory, he could threaten to kill them to gain even more concessions from L and the police. Mello had made serious enemies out of L/Light and Near alike, and he was confident he could take them both on. And Soichiro had to live with his role in all this.
