Twenty-four-year-old Marnie is at breaking point with her XXX-rated thoughts. For the past 3,723 days, Marnie has been experiencing an incessant barrage of graphic x-rated thoughts. Unbeknown to Marnie, she's caught in the grip of a remarkable form of obsessive compulsive disorder.

Marnie decides to attend meetings for people suffering from sex addiction, where she meets an unexpected kindred spirit in recovering porn addict Charlie. Later, things start to look up for her when she talks herself into a job opportunity at Ethel Magazine.

Marnie is determined to make the most of being single while living in London and jumps into bed with Benji, which has unexpected consequences at work. Meanwhile, Charlie takes a chance at rekindling his relationship with his ex-fiancé, Flora.

Marnie and Charlie try to pick themselves back up after hitting rock-bottom. Marnie gets help from an unexpectedly understanding Shereen while Charlie tries to prove he's up to his old job.

Encouraged by her new therapy, Marnie tries out trusting her old friend Helen with her secrets and brings her two worlds together at a big dinner party. What could possibly go wrong?

[Season Final] Marnie is shaken when she realises she's pushed Shereen too far, and starts to become paranoid that everyone has turned against her now they know about her condition.