Don is a single twentysomething who constantly fails to guide his way through life. His overactive mind doesn't help things as it plays out scenes of what he shouldn't say or do. When he moves into his recently deceased grandmother's house, he meets Eddie: his Gran's carer who doesn't want to leave. To help pay his rent, he decides to get a roommate in the shape of his teenage sweetheart...
Don has blown his chances with flatmate Sam and to make matters worse she has a new boyfriend. With no career, Don comes up with an elaborate story about a new job to try to impress her. Things finally appear to be going his way when a chance encounter with high-flying executive Marcus leads to a promotion. Of course being Don, it's not long before it all goes wrong.
Things are looking up for Don as he has begun to date a sweet girl called Jenny. However, her behaviour starts to ring alarm bells as her moods swing from adorable to violent in an instant, turning Don into a battered and bruised mess. To make matters worse nobody believes him. The tension comes to a head during the local pub quiz. Don and Jenny join Sam and Brian's team. They are determined to beat arch rival Derek. Jenny hates losing. Is this a chance for Don to show everyone her violent side?
Don and his boss Jason socialise together for the first time in the form of a double date with two posh girls they met at work. Don learns never to judge a book by its cover, as his date Felicity is far from prudish. Everything seems to be going well until Felicity invites them to her parents' country house for the weekend. It's a world away from Don's life and he struggles to fit in. He also has some over-protective parents to contend with.
Don finds himself having a bad start to the week when he notices that his hair is thinning, He ends up booking an appointment with a specialist to discuss having a hair transplant. Things get worse for Don when he notices that he isn't the centre of attention after he is completely ignored he storms out. After Eddie discovers Don's letter about the transplant he ends up assuming the worst. Eddie, Sam, Brian and Mrs Treacher attempt to support Don when they think that he doesn't have long left to live and Don ends up making the most of the situation.
Don ends up surprising himself after he takes pity on pretty homeless woman called Susan. Sam ends up suggesting that he ask her out. When Sam returns home with Brian, she discovers Susan sitting on the sofa in their living room whilst Don runs her a bath. The following day Don doesn't want Susan to leave but after he offends her on a day out he tries to make it up to her by getting to know her friends better.
Don learns that Brian is going to ask Sam to marry him and realises that he still has feelings for Sam. Eddie suggests that Don sees a therapist where he ends up taking the therapist into his confidence and confessing his feelings for Sam, as well as some slightly shocking habits. Sam decides to throw a fancy dress party to celebrate Brian's birthday. Don feels unnerved when he learns that his therapist has come along in fancy dress. Don tries to find him at the party to stop him from revealing any of his secrets.
After seeing the attention Eddie gets in his theatre club's production of Rain Man, Don decides to join up. The club puts on a tremendous production of Top Gun: The Musical. Meanwhile, Brian is still waiting to get a response from Sam about his proposal and is so desperate he even asks Don for advice.