He was a ghost who pretended to be a doctor in an attempt to stop the Ghostbusters.
On a Thursday, Dr. McCatheter entered the Firehouse under the premise he was a medical physician opening a new office in the neighborhood. He gave Louis Tully a free sample of special health chips, called "New Spirit Chips," that made humans allergic to ghosts and anything that had to do with them, including Ecto-1, saturated with ectoplasmic residue after years of busting. Two days later, when the Ghostbusters suffered the effects of the chips, McCatheter took over Ghostbusters and invited other ghosts to a party at the Firehouse.
Things were back to normal when Egon Spengler had Slimer, himself, and the other Ghostbusters locked in Janine Melnitz's apartment room in Brooklyn. After the ectoplasmic energy of the chips were trapped, the Ghostbusters were cured. Dr. McCatheter and his friends were eventually trapped by the Ghostbusters and Louis Tully. Peter Venkman and Louis confined McCatheter in Proton Streams before he could escape. Janine made so many sacrifices to the Ghostbusters, so after her apartment was wrecked, she paid them back by living in the Firehouse while her apartment room was repaired.
PersonalityIn his human form, McCatheter acts like an ordinary doctor, as he is very serious when diagnosing Peter Venkman to be allergic to ghosts. In ghost form, he is not unlike other ghosts and likes to cause mischief and throw parties with other ghosts.
Doctor McCatheter was one of the few ghosts to be slimed by Slimer.