Erin Gilbert (Kristen Wiig), a physics teacher at Empire State University, has her opportunity to receive tenure jeopardized when her old friend, amateur paranormal researcher Abby Yates (Melissa McCarthy), publishes a book they wrote together several years young, positing the existence of ghosts. Erin visits Abby and her new partner, nuclear researcher Jillian Holtzman (Kate McKinnon), to ask Abby to remove her name from the book, and is persuaded by Abby and Jillian to accompany them to the Aldridge Hotel to investigate claims of paranormal activity. Arriving there, the trio encounters the ghost of Madame Aldridge, the hotel's late owner, who vomits ectoplasm on Erin.Abby records the encounter, including a terrified Erin claiming that ghosts are real, and uploads it to YouTube and reddit, leading to Erin being fired. Abby then persuades Erin to join her and Jillian in opening a ghost-chasing business they call "Ghostbusters".
Meanwhile, subway worker Patty Tolan (Leslie Jones) runs into Rowan (Neil Casey), an introverted engineer who has created a machine that amplifies paranormal activity, intending to open a breach between the world of the living and the world of the dead and release an army of ghosts to destroy the human race. After an encounter with a ghost that Rowan releases on the subway system, Patty seeks the Ghostbusters and volunteers to become a member and lend her street-smarts to the team. They also hire a dim-witted receptionist, Kevin (Chris Hemsworth), and set up a base of operations in a Chinese restaurant and two vehicles, Ecto-1, a hearse lent to them by Patty's uncle; and Ecto-2, an old motorcycle.
Despite the government's attempts to discredit them, the Ghostbusters become increasingly popular as they capture more ghosts unleashed by Rowan, and ultimately confront Rowan himself, who chooses to commit suicide rather than let himself be captured. With his death, paranormal activity in NYC subsides, adding credence to the government's claims about the Ghostbusters. Going through Rowan's belongings, Erin and Abby learn Rowan attended the same school as them and his obsession with ghosts was born out of their own fascination with the subject.
Rowan returns as a ghost and swears revenge on the Ghostbusters, possessing Abby and attempting to kill Jillian. After Patty beats him out of Abby's body, Rowan possesses Kevin, steals the Ecto-2 and heads to Times Square, where he activates a second machine he had secretly built, opening a portal between the two worlds and raising an army of ghosts that begin wreaking havoc in New York City, while also powering up Rowan's ectoplasmic form. To display his dominance over them, Kevin/Rowan leads the ghosts in a performance of "You Should Be Dancing" by the Bee Gees.
The Ghostbusters intervene despite the government's attempts to stop them, venturing into the evacuated area in the Ecto-1, and Erin manages to exorcise Rowan out of Kevin's body. Rowan demands that the Ghostbusters choose a form for him, and Patty absent-mindely thinks of a spraypainted ghost she saw on the subway line. Rowan becomes a towering version of the graffiti ghost and begins rampaging.
The Ghostbusters fail to defeat him by crossing their streams, and instead Erin and Abby venture into the portal, with ropes tied around their waists, and trick Rowan into directing his forces into the portal. Jillian and Patty then close it, but manage to pull Erin and Abby out, while Rowan and his army become trapped in the world of the dead and Rowan's machine is destroyed.
The government once again discredits the Ghostbusters and they don't receive any recognition for saving the city, but manage to use their savings to buy a new base of operations in an old firehouse. Erin and Kevin develop a relationship, and while analyzing tape recordings during the battle against Rowan, during which paranormal activity was at its peak, Patty finds several cryptic warning about the arrival of Zuul.
