Mark Furness, once a successful architect, finds it increasingly difficult to control his "bad habits", such as climbing stairs in a strange sequence, swearing involuntarily and washing himself compulsively. His wife, Stevie asks him to leave their home, so he seeks solace with his best friends Nathan and Kathy, who take him under their wing. His life gets even worse when, at a meeting with his employers, he finds himself swearing uncontrollably and loses his job. Accepting that he is unwell, Mark visits a doctor, already convinced he is suffering from a brain tumour or meningitis. In the doctor's waiting room, another patient, Charlotte, recognises Mark's "habits" as symptoms of obsessive-compulsive disorder and Tourette syndrome, and invites him to a local self-help group she runs. Mark finds it comforting that others share his disorder and with Charlotte's help, hopes to face his problems and win his wife back. Mark tries to beat his disease and begins to make some progress despite the news that his wife is petitioning him for divorce. While meeting her to discuss terms - after covering himself in chocolate powder to combat his obsessive cleanliness - Mark and Stevie have sex. Mark interprets this as a sign that they will be reunited, but Charlotte warns it is nothing but "sympathy sex", leading to an argument between the friends. Mark storms off, but realises Charlotte was right when he sees Stevie with another man in their marital house. This plunges Mark into depression. His disorders deteriorate and, cutting contact with his friends completely, he moves into a rundown bedsit. His situation gets worse still when he begins to stalk Stevie, convinced she has begun a relationship with another man. Almost unrecognisable, he even attacks his friend Nathan on the street thinking that Stevie is having an affair with him. Nathan tells Mark that Stevie wants Mark to stop phoning and stalking her otherwise she'll call the police on him. At his lowest point, Charlotte re-enters his life and helps clean up his new home and his appearance. Charlotte then suggests they go for a drive to the seaside and Mark has her drive to the beach where Stevie's parents live in an attempt to try and find Stevie and the man she's seeing. But Charlotte scolds Mark insisting that Stevie is nothing but an "obsession" and that he has to move on. Mark Shows Charlotte a piece of architecture he admires and she reveals her talent for playing violin. The two share a kiss but Mark runs back to Stevie's parents home, upsetting Charlotte. Despite being turned out, Mark sneaks into the back garden where a party is going on. Seeing Stevie with her new partner he head-butts Gareth into the pool. Steve yells at Mark causing him to collapse in a state on the floor. Charlotte arrives to try and talk sense to Stevie who just ignores Charlotte's explanations. Charlotte slaps Stevie for her cruelty, in retaliation Stevie pulls Charlotte's long hair which is really a wig worn to disguise her hair loss from trichotillomania, she runs off in tears. Mark, finally seeing sense takes back the wig and Stevie tells him that the day they had sex, she only wanted things to go back to normal and have "Her Mark" back. Mark declares that he had always had OCD and will ways will and bids Stevie farewell. He finds a tearful Charlotte on the beach and recognizing her true worth, they begin a romantic relationship themselves.
