The narrative commences with an event involving the characters Shadow the Hedgehog and Postal Dude, resulting in their separation due to Shadow inadvertently falling through a trap door within a residence. Postal Dude, exhibiting resourcefulness, extracts a toilet as an improvised weapon; however, he subsequently succumbs to unconsciousness, engaging in an internal struggle with his malevolent thoughts, ultimately succumbing to their influence, thus transitioning into an antagonistic persona.

Upon regaining consciousness, Postal Dude, now referred to as "Other Dude" to denote his altered state, ventures into the hole to rescue Shadow. Meanwhile, Shadow engages in a confrontation with Hugo Strange, resulting in the incapacitation of the latter with broken limbs. Subsequently, a physically imposing incarnation of Eggman intervenes, rescuing Hugo Strange, and prompting Shadow to pursue them, leading to a showdown with Metal Sonic. Shadow emerges victorious, and together with the possessed Postal Dude, they confront Eggman.

Notably, Shadow harnesses a device to extract Eggman's life force, thereby augmenting his own power. This alliance then proceeds to ascertain the whereabouts of another malefactor, Zaz, whom they intend to confront. Their pursuit brings them into contact with Mr. Freeze, who amicably aids them. Their mission to apprehend Zaz leads to an encounter with Hugo Strange, who is administering therapy to Zaz. Mr. Freeze encapsulates Hugo Strange within a prison of ice, but Zaz delivers a devastating blow to Mr. Freeze. Shadow endeavors to assist the injured Mr. Freeze, while Postal Dude engages Zaz in combat.

Postal Dude displays commendable prowess in his skirmish with Zaz; however, Zaz ultimately releases Hugo Strange. Mr. Freeze retaliates by immobilizing Zaz's legs with ice, effectively incapacitating him. In a surprising turn of events, Postal Dude mistakenly inflicts fiery urination upon Mr. Freeze, inadvertently leading to his demise.

In the aftermath, it is revealed that Postal Dude had misconstrued Mr. Freeze as an adversary. Before Shadow can react, Hugo Strange's wheelchair transforms into a mechanized spider armed with miniature firearms. They swiftly neutralize this threat, with Postal Dude utilizing his lethal urine on Hugo. The ensuing conflagration engulfs the entire edifice, necessitating a hasty evacuation, fortunately undertaken by Shadow and Postal Dude.

Subsequently, they venture into the sewers to confront Killer Croc, only to encounter a formidable adversary known as V1 from Ultrakill. An intense and lethal confrontation ensues, ultimately culminating in Shadow forming an alliance with Killer Croc. Nevertheless, an unforeseen development arises: Postal Dude and V1 form a romantic bond, setting the stage for a climactic showdown—an epic duel pitting the newlyweds against an enigmatic hedgehog and a murderous crocodile.

Shadow, now unburdened by his inhibitor rings, unleashes his full power, inflicting a formidable beating upon Postal Dude and V1. However, V1 manages to mimic Shadow's abilities and delivers the decisive blow. Just as V1 is about to vanquish Shadow, Killer Croc intervenes, rescuing Shadow and retreating deeper into the sewers. This leads to a confrontation of good versus evil—Shadow and Killer Croc pitted against the homicidal couple.

Killer Croc and Shadow devise a strategic plan, with Shadow leading the way. Meanwhile, V1 and Postal Dude engage in a spree of violence against innocent bystanders. Ultimately, they plant a tracking device on Robin, which inadvertently leads them to the Batcave, where they intend to eliminate Batman. Following the murder of Robin, they opt to leave his lifeless body behind for Batman to discover, leaving him to grieve the loss of his comrade.