Charlie Chaplin's 20th film for Keystone marks a turning point in his career. From this point on, with one exception, he was to write and direct all his future films. In Laughing Gas Chaplin plays a dentist's assistant who is first seen entering the office officiously. The patients are fooled into thinking he is the dentist himself, until he picks up the spittoons and exits to a back room. He confronts a midget-size co-worker there. The Dentist finally arrives and the first patient is admitted. Laughing gas is administered, and the extraction performed, but the dentist is not able to awaken the patient. He sends Chaplin out to the pharmacy for an antidote. Chaplin encounters Mack Swain who is standing in front of the pharmacy, blocking the entrance. Chaplin gains entrance by performing some of his famous hat tricks, which non plus Swain. Exiting the pharmacy Chaplin gets into a fight with Swain which evolves into brick throwing, during which Swain and an innocent bystander, Slim Summerville, are both hit in the face, turning them both into dental patients. On his way back to the office, Chaplin encounters and flirts with the dentist's wife and accidentally tears off her skirt. When Chaplin arrives with the medicine, the patient has left, and the dentist has been called away to attend his distraught wife. Chaplin admits a beautiful female patient who he pretends to examine but with whom he flirts by grasping her nose with a pair of pliers and kissing her, to her apparent amusement. Summerville and Swain then arrive at the office and Swain catches sight of Chaplin in the back room. The dentist and his wife arrive and a melee ensues in which everyone is literally kicked out onto the pavement, except Chaplin and the wife who collapse in the waiting room.