The film begins in The Winchester, a traditional London pub. Shaun (Simon Pegg, his girlfriend Liz (Kate Ashfield) and her two friends David (Dylan Moran) and Dianne (Lucy Davis) are socializing while the overweight and foul-mouthed slacker Ed (Nick Frost) plays away on the slot machine. Liz is trying to encourage Shaun to strengthen their relationship and be more adventurous, rather than just reminiscing in the pub and hanging out with Ed. Liz reminds Shaun how she hasn't even met his mother yet, though they've been together for more than two years. The scene ends with Shaun staring vacantly towards the camera, clearly displaying anxiety.
The movie's intro credits roll, continually panning right through scenes of regular people performing everyday tasks. We see ordinary people moving supermarket trolleys, working behind tills (cash registers), waiting at the bus stop, or mindlessly listening to street music, all staring and acting zombie-like (a reference to the satirical elements of 'Dawn of the Dead').
Inside his house, Shaun wakes up and joins Ed in the living room where he joins in a video game on the PlayStation. Ed reminds Shaun that he has to go to work and Shaun begrudgingly goes to get ready. While Shaun is getting ready, his second flat-mate and college buddy Pete confronts him about Ed's behavior. Pete is fed up with Ed lounging around all day, freeloading off them and not bothering to find a job. Shaun defends Ed by saying he's harmless and fun to have around, to which Pete replies "Why? Because he can impersonate an orangutan? Fuck-a-doodle-do!". Both Pete and Shaun leave for work, with Shaun asking Ed to take down any phone messages (everyone's, not just his own) that come during the day. Just as Shaun leaves, Liz calls to leave an answerphone message for Shaun, asking him to book their restaurant dinner an hour earlier. Shaun doesn't hear the message.
The next sequence of scenes sees Shaun side-tracked by a series of bizarre events, including newspaper headlines describing attacks on members of the public, people collapsing at bus stops, and television news reports of more attacks and military involvement. Shaun arrives at work, an electronics retail shop, where he is manager. Shaun's step-dad, Philip (Bill Nighy), pays him a visit to remind him of his bi-monthly visit to his mum. The pair have a very frosty relationship and Shaun clearly does not like his step-father. Shaun holds a staff meeting for his team of clearly bored and feckless teenage staff. He tries to discipline a colleague for using his mobile phone on shop floor, and while doing this his own phone rings. To save looking hypocritical he is forced to pretend it's a work related call, and ends up ignoring a bemused Liz who's called to remind him about booking dinner earlier that evening.
