Louis B. Tully was Dana Barrett's neighbor across the hall in the infamous Shandor Building in New York. He had a job as an attorney, but got his law degree at night school and mainly handled taxes. Hopelessly in love with Dana, Louis would constantly make up excuses to see her, even going as far as ambushing her whenever she was coming or going from her apartment.
During a party thrown for his clients as a business expense, Louis suddenly found himself targeted for possession by Vinz Clortho, the Keymaster of Gozer. Although he initially escaped from the clumsy and inept Vinz, the Keymaster ultimately cornered him in Central Park and took over his body. Under Vinz's possession, Louis aimlessly sought out Zuul the Gatekeeper, who had possessed the body of Dana Barrett.After being inhabited by Vinz, he wandered around Central Park mumbling to himself and scaring off random strangers before clumsily mistaking a carriage horse for the Gatekeeper. The horse's coachman interrupted Louis as he was rambling, angering him and causing him to flare his eyes red and growl at him. Louis then ran off yelling threats about the coming of Gozer before getting arrested by police and dropped off at the Ghostbusters headquarters. Egon Spengler interviewed him and scanned his brain to reveal an image of a Terror Dog on his screen. The possessed Louis spoke giddily about the coming of Gozer before their conversation was interrupted by a phone call from Peter Venkman, who had just discovered Dana Barrett under demonic possession, to Egon. The following day a representative from the EPA came to the Ghostbusters headquarters and forced the shutdown of their protection grid, Temple of Gozer where their mated as part of the ritual to open the doorway into the spirit world and allow Gozer to enter the human realm. Upon concluding their union, Dana and Louis rested before taking their places atop the pedestals which previously contained the statues of their possessors where they awaited for the moment to open the doors to the Temple of Gozer. Once he and Dana completed opening the doorway they transformed into the grotesque demonic forms of their possessors, Vinz and Zuul, and took their places beside Gozer.
When the Ghostbusters destroyed Gozer by crossing the streams reversing the particle flow through the gateway, both Vinz and Zuul were burned to a crisp by the resulting blast. Louis and Dana survived however, and were extracted from within the Terror Dogs' burnt corpses by the Ghostbusters. Louis never knew that he and Dana had sex while they were possessed, and when he asked, she denied it.Five years later, as thanks for helping him, Louis served as the Ghostbusters' lawyer during their Judge Wexler's court. Louis proved to be a hopelessly inept lawyer, but the day was saved by the appearance of the Scoleri Brothers, whom the Ghostbusters caught after Wexler rescinded their restraining order.
Afterwards, Louis became the personal accountant for the Ghostbusters. During this time, he had an affair with Janine Melnitz and found a personal enemy in Slimer when the green ghost began hanging around the firehouse all the time. When the Ghostbusters went to battle Vigo the Carpathian, Louis, donning a spare uniform of Egon's, hurried to catch up so he could help them, encouraged by Janine. The moment at which he fired his particle thrower at the slime shell covering the Manhattan Museum of Art was the moment the Ghostbusters destroyed Vigo, causing the slime to weaken and be blasted apart easily by Louis, who was hailed as a hero by the crowd of onlookers.