A teenage Japanese boy wakes up to find himself alone in an empty room with no memory of his own identity or past. He surveys the room he is in and discovers he is a Japanese tourist and suddenly realizes the only thing he can remember is that someone was trying to kill him. While he explores the seemingly abandoned building, he is suddenly attacked again by the masked person from his memories and is told to fight with everything he has or die. Meanwhile, someone is remotely watching his actions. Deciding that he wants to live, he fights back and momentarily gains an advantage; as he attempts to make a killing blow, the mask cracks and his attacker is revealed to be a girl. Instead, he attempts to leave but finds himself surrounded by a desert and the girl, no longer attacking him and introduced as Ein, tells him he will now either live on as an assassin named Zwei or be killed. These scenes are interspersed with a flash forward of Zwei and Ein working together as "Phantom" on an assassination mission. Ein, poses as a maid, along with Zwei, manage to eliminate an entire household of armed guards and the head of a mafia boss in Las Vegas, once again proving their absolute power and indispensability to a mysterious organization called Inferno.

Ein conducts personal training to Zwei after agreeing to work with Inferno for several days, ranging from sparring in martial arts to small arms use in order to hone his skills as a potential Inferno assassin in upcoming Inferno-approved missions. Claudia enters the complex right after Scythe Master picks up Ein without revealing her presence and tells Zwei about the objectives of Inferno, as well as to give "hope" to Zwei that he can find out who he is before she leaves him with a goodbye. Meanwhile, the Scythe Master deploys Ein in an assassination mission somewhere in the United States to kill a person of interest.

After more training with Ein, Zwei is going to be tested. He has to kill a man named Captain Wallace of the Navy SEALs, who sold weapons to Inferno, but because they weren't his only costumers and he sold many weapons to terrorists. Wallace eventually sullied the name of Inferno and put them in danger of being suspected by the FBI after an FBI agent was found dead in Mexico, forcing him to beat Zwei and defeat him in a duel to the death in order to allow Inferno in rebuilding his life all over again. After hesitation of killing Wallace and almost being killed by him, his assassination skills awake and he shoots Wallace in his head after wounding him by using a combat knife to stab him in Wallace's right arm.

Ein and Zwei are given the order to kill Don Lucio, the boss of a mafia family based in Dallas, Texas as their first assignment together. Because he never leaves his mansion, the two Phantom assassins have one chance in either making the mission work or not. With the unexpected help of Scythe Master, who drove in to assist the two assassins in making their getaway, Ein and Zwei were able to successfully kill their assigned target and flee for safety. The two were later sent abroad to lie low due to the success of their mission. Zwei contemplates on how he had been indoctrinated successfully by Inferno as a now skilled hired assassin and killer.

Inferno tries to establish contact with a mafia family somewhere in Los Angeles led by Tony Stone, who was currently busting up individuals selling drugs at Melanie Square personally. During this time, Claudia invites Zwei over to her estate in order to act as her secondary bodyguard and a spy to maintain recon on the mafia itself. When the meeting for Inferno to conduct its influence on Melanie Square was not working in their favor as Tony refuses to do so, Ein and Zwei are deployed to wipe out the entire mafia and assassinate the wife and only child of Tony to prove that Inferno is a force to be reckoned with.

The surviving members of Tony's mafia plan revenge against Inferno by planning to release crucial information on certain mobsters who are suspecting of supporting Inferno from behind the scenes to discredit them and be eliminated from the inside. However, Inferno plans to conduct an assassination spree on all anti-Inferno mafia personalities in the West Coast of the United States with Ein and Zwei being in the forefront working with Lizzie to carry out the assassinations on the orders of its top leaders under insistence from Claudia. Thanks to their efforts, Inferno was able to consolidate their influence in the area.