Noises Off begins the night before a regional company of a new sex farce, Nothing On, opens for its first performance in Weston-super-Mare. Director Lloyd Dallas is trying to get his cast through a technical dress rehearsal, but they keep interrupting it with questions and problems. The plot of the first act of Nothing On is that the housekeeper of a country home, Mrs. Clackett (played by Dotty), is trying to enjoy some sardines in front of the television. She is interrupted by Roger Tramplemain (played by Garry), who is escorted by Vicki (portrayed by Brooke). Roger and Vicki want a private rendezvous and ask Mrs. Clackett not to tell anyone they’re there. As they sneak off to one of the bedrooms, Philip and Flavia (portrayed by Freddy and Belinda) arrive to hide from impending tax fraud charges. They likewise ask Mrs. Clackett to keep their presence a secret, and the result is a serious of mistaken identities, slamming doors, and lost plates of sardines. The act ends when a Burglar (played by Selsdon) is discovered by the group and turns out to be Vicki’s father.

The first interruptions in the rehearsal come from Dotty, who cannot keep her lines straight and when to pick up and put down props. Dotty has funded the production and given herself a starring role. The next interruption comes from Garry, who expresses concern about the lack of rehearsal time. When Freddy and Belinda make their entrance, their door will not close while another one upstairs won’t open. Lloyd asks Poppy, the assistant stage manager, to find Tim, the stage manager. Tim has not slept in two days because he was building the set and begins working on the doors. During the break, Lloyd and the cast become concerned that no one can find Selsdon. Poppy looks for him and worries that an old man the police found in the gutter might be Selsdon. During this explanation, Selsdon arrives and is told to wait backstage for his entrance. Meanwhile, Belinda reveals that Garry and Dotty are having a romantic relationship.

After the play resumes, Freddy interrupts again because he wants a motivation for taking some props offstage. When he persists, Lloyd blows up at him about the lack of time remaining. Belinda tells Lloyd that Freddy’s wife left him, so Lloyd gives Freddy the motivation he requested. Later, the rehearsal halts because Brooke loses one of her contact lenses. In the ensuing search, Poppy gets knocked in the head. Freddy, who is extremely sensitive to physical violence, gets a nosebleed and feels faint. When the play resumes, Selsdon is late for his entrance and the cast must retrace its steps. They finally rehearse a larger piece of the play...