Summary:

One day during a Sparda memorial ceremony of some sort celebrated by people of the Order of the Sword, Devil Hunter Dante came crashing through the ceiling and killed the leader of the Order. It turned into a panic and everyone fled to safety except for Nero, a member and warrior of the Order, who stayed to fight the mysterious assassin.

Unfortunately for Nero, Dante 'escaped' after their battle and he lost track of him. Credo, the Supreme General of the Order and older brother to Kyrie, ordered him to find Dante, and so he set off on a journey to look for this murderer.

Unknown to most, it was around this time when Sanctus, the leader of the Order who was just murdered, revived thanks to the "Ascension Ceremony" (explanation on what it is wasn't given) with the help of Credo and Agnus,  Chief Alchemist of the Order.

As Nero continued to travel, he came across Agnus's lab and met him for the first time. Agnus planned to kill Nero, though, and they battled only to result in Agnus's defeat. Nero demanded Agnus for answers on what was going on, but he got careless and ended up getting caught and was going to be used as a subject of experiment. So far, the only answer he got was that Credo was behind all this.

While Nero was knocked out, he had a flashback of the time Kyrie nearly got killed by a demon, which was the time when his arm mysteriously changed into this devil arm that he possessed. Immediately after the flashback he suddenly woke up and gained control of the now-resurrected Yamato sword that had been kept in the lab all this time (reason unexplained). The 'devil' within Yamato awakened once Nero held it and he used its power to force Agnus into retreating.

Having freed himself, he continued his journey to find Credo instead of Dante to seek out his answers. He eventually found him and was going to ask him what was happening and why when Credo attacked him and turned into his demon form, forcing Nero into a battle and attempting to kill him as ordered by Sanctus. Just after Nero defeated Credo, Kyrie came into the scene to witness Nero's demon arm. Nero tried to explain but Agnus appeared to interrupt and kidnapped Kyrie as, again, Ordered by Sanctus.

And so Nero continued forth to look for further answers and to save his girl. He soon bumped into Dante who asked him to return him Yamato which belonged to the Sparda family. Refusing to comply, they ended up battling but in the end Dante let the kid keep the sword anyway and let him go.

Nero eventually found Sanctus who stood on top of a giant statue known as the Savior that looked similar to Sparda. Now knowing that he was behind all this. Nero leapt at Sanctus to attack, but Sanctus revealed Kyrie from within the Savior which promptly stopped Nero from attacking. Sanctus then summoned the Savior to grab Nero in its hand, immobilizing him and stealing the Yamato sword in his hand.

Just in the nick of time, Credo came to help and save Nero and his sister since he realized that Sanctus had betrayed him by using his sister. Alas, he failed to take Sanctus down and Dante and Trish came into the scene to see Nero and Kyrie sucked into the Savior to form his core. It was during that time when Sanctus revealed that Nero had the blood of Sparda, which was needed to bring life to the Savior, not that anyone reacted to that revelation.

However, now that the hero is in trouble, it is finally Dante's turn to save the day, but Sanctus and the Savior had flown back to Fortuna City to commence chaos. As he ran after them, Dante destroyed the hell gates that Agnus created using the power of a few devil arms. Knowing that Yamato was also still in the hands of the enemy, Dante traveled back to retrieve it, killing Agnus as a prerequisite. Eventually, he caught up with the Savior and destroyed its core with Yamato, releasing Nero who was trapped inside.

As Dante continued to battle the Savior outside, Nero fought Sanctus in his demon form inside and defeated him. Having rescued Kyrie, he got out of the fallen statue to see Dante greeting them. Nero thanked him for the help and was going to return Yamato to him when Dante decided to entrust it to him and left, leaving the happy couple alone now that he had finished the job Lady got him.