Agents Herschler, MacMillian, and McGee were assigned to the case of Jeffrey Chance, a moderately successful writer of mystery novels. Jeffrey and his wife, Susan, had been haunted for several months by what Jeffrey claimed was the ghost of his twin brother, Randall. The Agents interviewed Jeffrey and Susan Chance in their home in Burdrock Mills.
Jeffrey explained that he had never been on good terms with his brother, who had always been the "black sheep" of the family. Two years ago, Randall moved to the City and began contacting Jeffrey, asking for money. Jeffrey tried to put his brother off, but soon it became apparent that Randall was stalking Susan. Several months ago, Randall broke into Jeffrey's home and attempted to murder him. He hit Jeffrey on the head with a marble bookend, and would have killed him if Susan had not then shot Randall through the heart with Jeffrey's revolver. Jeffrey was in a coma for several days. Although he has recovered from his injuries, he has not written anything since the attack. The hauntings began just a few weeks later, with Randall often appearing to Susan and pretending to be Jeffrey, insisting that she had shot the wrong man.
Susan requested specifically that the Agents not merely remove Randall's ghost, but permanently dissipate it. They promised to bring the issue up with their supervisors, and left to perform some background research.
The Agents were immediately suspicious. They discussed several possible theories, including that Randall was actually possessing Jeffrey's body without Susan's knowledge. Or, perhaps, Susan and Randall conspired together to kill Jeffrey, and Randall was now impersonating him. They decided to split up. Herschler and McGee went to spy on the Chances at the hotel where they were staying during the investigation, while MacMillian returned to the Chance's home to try to locate the ghost.
MacMillian found the ghost in Jeffrey Chance's study. As she suspected, the ghost claimed to be Jeffrey himself. According to him, Randall's ghost had slipped into his body while he lay in a coma. He demanded that MacMillian help him return to his body.
At the hotel, McGee briefly possessed Susan Chance and discovered that she was pregnant. Meanwhile, Herschler attempted to possess Jeffrey but was forced out — exactly as though Jeffrey was already possessed by another spirit. This seemed enough evidence to conclude that "Jeffrey" was not who he seemed to be, and that Susan Chance might well be in danger. Agent Herschler, thinking he might need help driving the invading spirit from Jeffrey's body, rushed back to Headquarters to get backup.
MacMillian and the ghost calling himself Jeffrey arrived at the hotel, followed shortly by Herschler, and Agents Doug Sands and Walter Hanley, who accompanied Herschler in physical form. MacMillian changed her form to resemble Jeffrey and manifested visibly, hoping to gauge Susan Chance's reaction. Herschler attempted to possess Jeffrey's body again and this time succeeded, forcing out a spirit that looked identical to the first. At this point, two things happened:
First, Susan, seeing Jeffrey's body slump momentarily as Herschler took it over, called out, "Randall!"
Second, Herschler took a deep breath and found himself wheezing through smoke-afflicted lungs. Jeffrey never smoked, but Randall had been a smoker since he was 13.
Clearly, something was amiss with everyone's story. MacMillian used her beckoning power to immobilize one of the twin ghosts, while McGee teleported the other out of the room. Herschler then proceeded to interrogate Susan Chance.
She revealed that she had been having an affair with Randall Chance. They had planned Jeffrey's murder carefully: Randall would kill him, then assume his identity, claiming to have killed "Randall" in self defense. However, Jeffrey managed to get the better of Randall, bashing him in the head with the marble bookend before Susan killed him with his own revolver. Hoping to avoid suspicion, Susan told the authorities that she had killed Randall, and that the man in the coma was her husband. Little did she know that it actually was her husband, or rather her husband's ghost possessing his brother's body. When he awoke from the coma, Jeffrey fooled his wife into believing he was Randall, while to the general public he pretended to be Randall-pretending-to-be-Jeffrey. He was simply biding his time until his brother's ghost was destroyed... after which, he planned to murder his unfaithful wife.
At this point, Senior Agent Walter Hanley immediately closed the investigation and ordered all Agents back to Headquarters. Randall's spirit was allowed back into his rightful body, Jeffrey's ghost was released, and the three conspirators were left to work out their grievances amongst themselves.
A few days later, Hanley brought the Agents a newspaper clipping: Susan Chance had fallen from an upper story window in their home and was killed. Police were still searching for her husband, who had vanished the same day. Although it was widely believed that Chance had not written anything since his coma, police found an unfinished manuscript hidden in a false cabinet in his study. The story was about a man whose wife was having an affair with his twin brother. He kills them both in a fit of rage, but then realizes that he has amnesia, and cannot remember which brother he actually is. The manuscript is titled A Mirror Darkly.