Marc Spector was born on March 9, 1987, in Chicago, Illinois, The son of a rabbi to Elias and Wendy Spector. One day during his childhood, Spector and his younger brother Randall adventured into a cave near their home which unexpectedly flooded due to heavy rainfall, resulting in Randall drowning. Consumed by the loss of Randall, Wendy blamed Marc for Randall's death and became physically and verbally abusive to him as well as becoming an alcoholic. The trauma from Randall's death and his mother's abuse caused Marc to develop dissociative identity disorder (DID), manifesting the alter Steven Grant; Grant is based on a fictional archaeologist from the movie Tomb Buster that he and Randall used to watch together. Grant would remain unaware of his host's abusive childhood, as Spector wanted to ensure he remained happy and safe from the harm he had to endure. A teenaged Spector would eventually leave home despite Elias' attempts to convince him that he would find help for Wendy.

  In adulthood, Spector joined the United States Military, but was dishonorably discharged for going AWOL after entering a fugue state while on duty. Spector later began working as a mercenary with his former commanding officer, Raoul Bushman. During a job with archaeologist Abdallah El-Faouly at a dig site in Egypt, Bushman was overcome with greed and executed all of the accompanying archaeologists, including El-Faouly. Spector attempted to save them, but was shot and left for dead by Bushman. Mortally wounded, Marc dragged himself to a nearby temple erected in honor of the Egyptian God Khonshu and prepared to commit suicide. Khonshu himself would intervene, granting Spector a second chance at life in exchange for becoming his avatar on Earth and carrying out his wish to exact vengeance on criminals. Spector agreed, with Khonshu resuscitating him and bestowing an armored suit and various powers in return, turning him into the vigilante Moon Knight.

 As Moon Knight, Spector dutifully served Khonshu and killed many criminals and evil doers under his name. Remorseful over El-Faouly's murder, Spector met his daughter Layla but was unable to tell her the truth of her father's death. Spector and Layla eventually married, with Layla accompanying him during his ventures as Moon Knight. Suspecting that Khonshu was planning to make Layla his next avatar, Spector separated from her to ensure her protection.

  Spector survived the cataclysmic events of the Blip incurred by the alien warlord Thanos in early 2018,[2] and had issued a new passport that December.[3] By 2025, at Wendy's shiva, Spector was unable to bring himself to enter his old home to attend, despite his father's urging. Out of grief and trauma, Spector allowed Grant to assume general dominance over his body, who continued to remain unaware of his status as an alter to his host.

Stopping Arthur Harrow

 Steven Grant meets his other self

 Two months later, Grant had relocated to London, England and began working as a gift shop assistant at the British Museum. However, Grant is still susceptible to "blackouts", during which Spector reassumes control while Grant remains suppressed in his psyche. One night, Grant attempts to keep himself awake, only to accidentally fall into deep slumber and wake up at a village near the Austrian Alps, now possessing an ancient scarab. He is promptly pursued by enemy forces, retreating deeper into the town where he meets religious zealot Arthur Harrow, who demands the scarab from Grant. Grant escapes in a cupcake truck while being pursued by Harrow's enforcers; he is unexpectedly saved by a mysterious, disembodied voice communicating to him. Grant eventually wakes up during the ordeal to find himself back in his flat. Although initially relieved, he soon comes to the realization that two days had passed since he "blacked out" upon noticing several oddities around his residence and being scolded by his mis-scheduled date. Returning home that night from work, he notices a storage facility key and a cell phone hidden in his flat, and calls Layla, who berates him for ignoring her calls over the course of several months. The following day, Grant is confronted by Harrow during his shift, with several witnesses and security personnel at the museum revealing themselves to Steven as Harrow's followers, and that they all serve under the Egyptian demoness Ammit. Later that night, in pursuit of the scarab, Harrow summons a jackal monster to attack Grant at the museum. Grant runs from the monster, but gets cornered in the bathroom. Spector emerges to offer his help to Grant subconsciously, with the initially reluctant Grant obliging, prompting his transformation into Moon Knight, who proceeds to savagely beat the monster to death.

Reconnecting with Layla El-Faouly

 The following day, Grant returns to the museum, only to be sacked following a review of the security footage from the previous night's attack, which does not show the monster or Moon Knight and only Grant "trespassing" after hours. He then heads to a storage facility and uses the key that he found in his flat where he discovers Spector's belongings, including the scarab he escaped with hidden inside a duffel bag. Just then, Spector himself "appears" in a reflection before Grant, revealing himself as Khonshu's Avatar. Frightened, Grant flees the facility after encountering Khonshu, only to be met by Layla El-Faouly, who introduces herself as Spector's wife. The two of them go to Grant's flat where he is apprehended for stealing the scarab by Police Constables Billy and Bobbi, who are secretly followers of Ammit serving as Harrow's disciples. Harrow brings him to their cult's hideout and demands the scarab from Grant, which Layla shows up with. Aggravated, Harrow summons another jackal monster to go after them. Just as it pushes Grant off of the rooftop of the hideout, he summons his own suit, much to Spector's bewilderment. The jackal successfully overpowers Grant, tiring him physically, and causing him to reluctantly converse with and relinquish control to Spector, who dons the Moon Knight armor and kills the beast. However, Khonshu soon berates Spector for losing the scarab to Harrow amidst the encounter, and threatens Spector with the prospect of claiming Layla as his new avatar, before sending him to Egypt.

Locating Ammit's tomb

 In Cairo, Spector attempts to track down a lead to Harrow's current whereabouts to no avail, while he and Grant continue to experience unusual momentary blackout episodes during which a third alter whose existence is unknown to Spector and Grant, Jake Lockley, takes over their body. After losing one of Ammit's worshippers to suicide, Khonshu causes a solar eclipse, intended to call for a council hearing of the Ennead and their respective avatars. Spector is summoned to the Great Pyramid of Giza, where Khonshu projects his accusation of Harrow for attempting to seek out Ammit's tomb and resurrecting the demoness, to which the accused falsely denies when summoned.

 Spector and Layla pay a visit to Anton Mogart who is in possession of the sarcophagus of a medjay who knew where Ammit's tomb was. Before they can use the cloth map to locate the tomb, Harrow arrives and destroys the sarcophagus. After a fight with Mogart's people, Spector and Layla make their way into the desert, but are unable to put the map pieces back together. Spector reluctantly lets Grant take over and he quickly reassembles the map of constellations. Khonshu turns the night sky back in time to the night the map was made so that the star locations matched. As a result, Khonshu is imprisoned in an ushabti, causing Spector and Grant to lose their powers.

Finding Ammit's ushabti

 After escaping Harrow's men, Grant and Layla use the map to find the tomb. They grab supplies from Harrow's empty camp and find an alternate way into the tomb after kissing each other, upsetting Spector. They encounter mummified monsters and Grant splits off from Layla as they both try to escape. He comes across a sarcophagus and realizes that it is likely Alexander the Great's after recognizing the Macedonian language. He opens the sarcophagus and reaches down the mummy's throat where he finds Ammit's ushabti. Layla arrives and confronts Spector over what happened to her father. Harrow and his followers arrive and after Spector quickly kills a few of them, Harrow shoots Spector twice in the chest, killing him.

The Duat and psychiatric hospital

 Spector wakes up in a psychiatric hospital, populated by people in his and Grant's lives. Spector then finds himself in an office, where Harrow appears as a psychiatrist seemingly helping him. Spector escapes and finds a room containing two sarcophaguses, one containing Grant in a separate body. Spector and Grant embrace, leave the second sarcophagus–which contains Lockley–and are greeted by a hippopotamus-headed woman, who causes Spector and Grant to scream. After this, Spector begins to switch between two different realities: encountering the woman with Grant, and in the office with "Dr. Harrow".

 In the first reality, Grant identifies the woman as goddess Taweret, who explains they are dead and the "psychiatric hospital" is a boat sailing through the Duat, the Egyptian afterlife. Taweret weighs their hearts on the Scales of Justice in order to determine if they can enter the Field of Reeds, but discovers their hearts are imbalanced by hidden memories in the "hospital" she suggests they explore. Grant discovers Randall's death and Wendy's abusive actions, while Spector shows him a memory of how he became Moon Knight and Khonshu's avatar on a mission with Bushman. Spector and Grant try to convince Taweret to let them return to the living world, and she steers the boat towards the Gates of Osiris. Spector further reluctantly explains to Grant that he unknowingly created him as a result of Wendy's abuse, and they both reconcile with each other. The scales fail to balance, causing hostile spirits to attack the boat and push Grant overboard, freezing him into sand.

 In the second reality, Spector appears in "Dr. Harrow"'s office, who tries to get Spector to calm down by saying he did not shoot him and that his mind is "violently vacillating" between the realities. Harrow tells him that he has made a reassuring fantasy that he is a superhero, but is repeatedly told that he is not a doctor. Harrow recaps how Spector said he arrived to the office, specifically that he had an encounter with a talking hippopotamus, asking him if he thinks that is sense or nonsense, which Spector answers him as nonsense. Harrow asks him to continue talking about the little boy that he was talking about before his screaming, but Spector grabs the paperweight from Harrow's desk, telling him he feels great now, and Harrow buzzes in the orderlies to sedate Spector. As a result, he returns to his conversation with Taweret. When Grant tells Spector that if he does not show him the memory that he is hiding, then that will keep their scale unbalanced and everyone who dies, including Layla, would be his fault. Spector begins repeatedly hitting himself in the head, saying that Grant "can not make him", which returns him to Harrow's office. Harrow tells him he is proud of him for being able to relive his traumatic memories, and asks him whether he thinks he created Grant to hide from the awful things he's done or if he thinks Grant created him to punish the world for what his mother did to him. Harrow tells him that the only way to truly know, is to open up to Grant. After Spector confronts Grant about their mom being dead, Grant's denial brings him to Harrow's office. Harrow offers to call his mom for him but is stopped when Grant accepts his mother's death.

 After Grant freezes, the scales become balanced, and Spector finds himself in the Field of Reeds. He decides to leave it to pursue Grant, giving a heartfelt speech to him with his heart, and freezes alongside him. The Gates open and Spector and Grant are resurrected with the help of Taweret, while Layla frees Khonshu from his ushabti; Khonshu fights the now-released Ammit.

Battle of the Gods

 Spector and Grant awaken back in their body, which causes Khonshu to sense their return and bond with them again, healing their body and restoring their powers as Moon Knight and Mr. Knight. Layla discovers they can defeat Ammit by binding her to a mortal body. As Harrow, Ammit, and their followers begin judging everyone in Cairo, Spector, Grant, Layla as Scarlet Scarab, and Khonshu arrive to engage them in battle. Spector and Grant work together in fighting Harrow as they seamlessly switch, until they are overpowered by Harrow. Harrow almost kills them, but they experience a sudden blackout and reawaken to see Harrow brutally defeated by a third identity.

 Spector and Layla are able to seal Ammit in Harrow's body, imprisoning her, and Khonshu urges Spector to execute Harrow and Ammit. Spector refuses and orders Khonshu to release him and Grant from being his avatar, which he does. The pair find themselves in "Dr. Harrow"'s office, where they reject the vision and choose to continue their new life together and wake up in Grant's apartment in London. Later, a crippled Harrow is delivered to a limousine before being confronted by Khonshu. Khonshu introduces Marc's other identity Jake Lockley, who then shoots Harrow.