Several years after Sigma was defeated at the hands of X along with Zero's help, the world of reploids began to breath easier for the first time in years. A brilliant reploid scientist by the name of Dr. Doppler claimed to have developed a cure for the Maverick Virus using his Neuro-computer (to suppress any maverick tendancies). As more reploids gathered around Dr. Doppler the population grew and a new town was formed. This town was named Dopple town in honour of Dr. Doppler and his work.
Everything seemed to be going well for Dr. Doppler. Until suddenly mavericks appeared in the town and began to attack. The evidence Maverick Hunter HQ had gathered showed that Dr. Doppler was behind it all. X and Zero were dispatched to stop the mavericks and apprehend Dr. Doppler. As they made their way to catch Dr. Doppler their HQ came under attack by maverick forces and they were ordered to return.
X and Zero returned back to the base at once. When they arrived they found the base nearly overrun by mavericks. Zero stayed outside to handle their outside forces while X went to work clearing out their base. As X was making his way through the base, he encountered a Hunter by the name of Mac. As X tried to ask him how it had happened, Mac turned on him and used a paralysis attack to stop X. Mac, having captured X, called in a lifter mechaniloid to take X away.
By this time however, Zero had finished sweeping the perimeter and clearing up the remaining forces outside. Zero entered the base and noticed that parts of it hadn't been swept yet. Immediately he knew something was wrong and searched for X. He found X being held captive by Mac, who had turned traitor, and was planning to turn X over to Dr. Doppler. Zero fought Mac and easily defeated him and freed X.
Zero and X split up again, with Zero heading back down to clear out the rest of the base while X proceeded up to the roof to take care of one of the larger Mechaniloids that was causing problems up there. X defeated the large mechaniloid fairly easily without too much damage to Hunter HQ.
After defeating the Mechniloid, X proceed back to Dopple town to deal with the rioting mavericks they hand't gotten to before.
Once X had dealt with a few of the mavericks a new maverick appeared to attack him. One, sent directly by Dr. Doppler, named Bit. Bit challenged X but was no match for X's superior capabilities and was deftly defeated. Soon after Bit's sibling reploid, Byte, challenged X and received the same fate.
To make matters worse for X, an old enemy had also been resurrected… Vile. Given new powers by Dr. Doppler, Vile instead of assisting Dr. Doppler chose to attack X. X managed to defeat Vile again, this time without Zero having to sacrifice himself.
Soon all the mavericks rampaging in Dopple Town had been cleared and X began to hunt for Dr. Doppler.
X was able to locate Dr. Doppler and soon confronted the mad doctor in his lab. After defeating him in battle, Dr. Doppler snapped to his senses and revealed that Sigma's true form now was that of a computer virus. And through that virus form, he'd been controlled by Sigma. And furthermore that he'd built Sigma a new battle body. He also provided X with the location of the body and urged him to destroy it before it was too late.
X hurried to where the body was located, but was too late to destroy it. Sigma had already taken possession of it. X and Sigma fought once again. With X yet again defeating him. Though once his body was destroyed, Sigma shed his body and reverted to his virus form. He claimed that if he could not kill X or make X join his cause then he would simply take X's body for himself.
X fled from Sigma, running to the end of the base to escape. Unfortunately he only reached a dead end, with no way out… Sigma closed in. Ready and fully intent to possess X and take his body for himself. Until the ceiling cracked and a section gave way. Zero dropped in and using his saber he slashed Sigma. He explained that Dr. Doppler had worked up an anti-virus to counter Sigma and then loaded it into his Saber. The anti-virus took it's course and destroyed the digital representation of Sigma.
But knowing that Sigma existed now as data, X and Zero realized that getting rid of Sigma would not be so easy and that he would likely return again.