Summary An explosion in the background goes off as X proceeds through the power plant. Spark Mandriller continues to watch X's progress using surveillance cameras, commenting that X has almost reached a place where something is waiting. X rushes into an empty room, and is attacked by a slime ball from above, immobilizing him. X looks upward, and sees the Thunder Slimer waiting in ambush. X remembers hearing about the Thunder Slimer, recalling it being a secret experiment that was supposedly disposed of when it was declared too dangerous. The Thunder Slimer speaks, saying it will not forgive whoever threatens its master before firing an electric blast downward at X. The Thunder Slimer continues firing at X who could only dodge, but has his feet caught by a slime ball. The Thunder Slimer them drops down, tackling X. Getting back on his feet, X wonders why the Thunder Slimer didn't finish him off using electricity, and notices the Thunder Slimer has connected itself to the ceiling to recharge. X fires at the Thunder Slimer, but his buster shots get absorbed by the thick slime coating. X rushes up a wall and throws himself onto the Thunder Slimer, endures the electrical charge going through him as he fires a charged shot right into its physical core. The Thunder Slimer says it would never be defeated for its master and explodes, sending Rock falling to the ground. X gets back up, but sees what little remains of the Thunder Slimer throwing itself at X. X blocks it with his arm, as Thunder Slimer says that it pledged loyalty to its master because it saved it from disposal. As Thunder Slimer's power weakens, its words become slower, and it eventually falls off X's arm and onto the ground. X laments the loss for a brief moment before continuing forward.

Going down a narrow corridor, X reaches an electrically charged gate which he forcefully opens, and sees Spark Mandriller waiting for him. Mandriller says that X had a child-like face back when they were in the same unit and that he has grown a bit, and says that he wished the "Slime Thing" would have put up a better fight but is no more than trash now. Angered, X asks Mandriller this is something to call a subordinate who died for him. Mandriller rushes towards X with a punch, saying that the only thing that matters is loyalty and being able to take some responsibility. X is sent flying across the room and towards the wall as Mandriller prepares a Electric Spark attack that flows through the ground and hits X. Mandriller hits X with a second Electric Spark attack and punches him into the ground. Bragging that X was nothing more than trash for being defeated, Mandriller turns his back on X and is shot in the back. Mandriller turns back to X, angered that a new-blood like X is able to hit him. X says that loyalty towards a demon like Sigma is blind. Mandriller shouts that X's words alone can't defeat him, and fires an Electric Spark. X attempts to dodge it by climbing the wall behind him, but the Electric Spark follows him upwards, hitting him. X gets back up and looks around, but cannot find Mandriller. Climbing up on the cable vines above, Mandriller drops down on X kicking him down saying that X can't stand after such a brutal attack, but X fires a shot that destroys Mandriller's chin piece. Angered further, Mandriller charges at X with a punch as X changes weapons as he declares that if Mandriller fights out of loyalty, then he would fight for peace. X fires a Shotgun Ice blast at Mandriller, freezing his arm. X recalls that Zero taught him about getting enemy special weapon chips and installing them in his buster. Mandriller still shocked that X just used Penguigo's weapon and his sudden grown, realizes that X could be the legendary Repliroid "Rockman".

Upon realizing this, Mandriller loses all will to fight, and begs X to save him and to fight Sigma together. Before he is able to tell X where Sigma is, Mandriller is killed by a shot to the back as a hologram of Sigma says that he has grown tired of Mandriller's boasts of loyalty. Sigma says that X is someone who also seeks power, and offers X to join him. X refuses, and destroys the armed hologram projector. As Sigma's hologram fades, X salutes the fallen Spark Mandriller, saying that Sigma will not be forgiven.

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Commentary The Spark Mandriller arc concludes without much direct consequence to the main story for the most part, though quite a few details have been introduced. Thunder Slimer does indeed gets some backstory here, and is revealed to be a scrapped top-secret experiment of sorts, even though it was never explicitly mentioned what this experiment was meant to be for. It was also given some personality, being quite literally a fiercely loyal pet to Mandriller who in turn disregards it as no more than mere scrap when it failed to stop X. As for Spark Mandriller, his aggressive and arrogant personality continues the sequence of Special A Rank Hunters under Sigma, putting them all in a negative light up to this point. There also seems to be some personality theme behind each boss character so far; while Penguigo puts his faith in "Rank", Mandriller seems to put his in "Loyalty". Whether this is a case of each boss having some kind of unique quirk or a narrative cliché, I find this theming helpful to test X's personality as a character going through development, which I believe was the intended purpose. Mandriller cowardly switching sides after he was defeated just shows how 'brittle' his own convictions are, further showing how defeated he has been against X.

Perhaps one of the biggest reveals in this chapter in the name drop of "Rockman" (an obvious nod to the titular hero of the classic series), which Mandriller referred to as the title of a legendary Repliroid known for its ability to constantly grow and develop, attributes that X certainly is showing so far. While Mandriller calling "Rockman" a Repliroid isn't technically correct, I'm going to assume that this is due to its status as a legend (and I doubt that anyone in this era would know how things were back then). Otherwise it is an interesting call-back and could even be a nod to X being mistakenly referred to as "Rockman X" (or "Mega Man X") in some cases.