Young Dylan is told by a doctor that he has the STI chlamydia and advises him to inform his past girl-friends. Starting alphabetically he chooses Abigail and recalls how he went to a wedding with his friends Evie and Luke. Helen the bride is angry that her groom Angus once dated Evie and wants to get back with her so to help Evie Dylan assures Helen that she is the only one. Dumped by his plus one he befriends the female vicar but ends up bedding the receptionist - Abigail. He must now contact her with his unwelcome news.

Having split with Helen Angus is now crashing with Dylan whilst Evie is engaged to Mal. Eighteen months earlier the sex-mad Luke persuaded Dylan to make up a foursome so that Luke could bed student Cleo, pairing Dylan with the older Anna. The two women visit for a dinner party but there is a power cut and Anna and Dylan end up in bed together, though she tells him he is too young to be a permanent boyfriend. In the present he rings her with his bad news - unaware that she is at a conference on speaker-phone - and then gets a call from Abigail.

In 2013 Luke persuades Evie and Dylan to accompany him to the birthday party in the Scottish highlands of his dream girl Ilona and eventually beats rival Ivan into bedding her though she has a bitter surprise for him. Dylan meanwhile sidesteps business woman Cressida but ends up being forced to strip nude by Ilona's gay brother, who has seen him prang Ilona's car. Still nude Dylan gets it together with Evie, resulting in her also having to make a trip to the clinic. Afterwards Dylan runs into Abigail and is relieved to hear that she is fine.

On a bench with Abigail Dylan recalls Evie's birthday party from three years earlier. Here Luke tries to pair her with the annoying Jonno whilst Dylan has reckless sex in a toilet with new girlfriend Jane, resulting in them both ending up in the casualty department. Dylan is upset to learn that Jane merely regards him as one of several lays whilst a romantic picnic leaves Evie and Jonno locked out on the hospital roof. Jane does however agree to be Dylan's 'plus one' at the forthcoming marriage of Angus and Helen.

Dylan visits another former girlfriend, Bethany, now a housewife and mother. Four years ago he is dating her but falls foul of her over-protective brother Gareth, who views all her suitors harshly. At the same time the charmless Luke is being duped by female work colleagues on a bowling alley outing into humiliating himself with boss Diana who claims he did something to her he cannot remember. Evie is with them but her camera is stolen and a call to Dylan gets him out of an awkward dinner party with Gareth as he races to lend her his. In the present Bethany notes that...

Dylan has another date with Abigail but Luke persuades him to look up another ex-girlfriend, Phoebe, from five years ago at which time Evie was going out with Angus. It becomes obvious during a pub quiz that Phoebe is more suited and Dylan tries to explain to her that they should split up but she gets in first. The boys visit Phoebe's mum from whom they learn that Phoebe is dead. Whilst visiting her room they brawl, explaining to Phoebe's mother that they both loved her daughter. Luke urges Dylan to tell Evie how he feels about her and, when Dylan bottles it, Luke ...