What came before
You really should read 'Best Sister Ever' first, you know. But it case it's been a while, here is a brief summary.
SPOILER ALERT
Martin, 40 years old, owns an IT company. When overseas clients from Mexico and the Philippines fail to pay his invoices, he goes bankrupt. Everyone is fired, including his trusted secretary Annabelle. His marriage to Monique ends too, as she is no longer interested when he can't offer her the life to which she is accustomed. After having paid all his bills, he is left with just 3000 euros in the bank, a massive debt and no job.
His sister Kate, 24, shows up to his rented house and takes him with her to London, where she owns a tiny house in Southwark. Kate works for Keller and Fox, a talent agency. Her work frequently takes her abroad. They share a bed, for lack of space. Since they enjoy each other's company, Kate signs them up for dancing lessons.
Kate gets Martin a gig in a TV-commercial, starring as a butler to an old lady. That old lady is played by Diana, 51, who in reality looks a lot better than in the ad. Martin also meets Melody, 26, who works in TV as a make-up artist and production assistant. When Martin gets upset after a kissing scene, Melody offers to kiss him for real. He declines, not wanting to appear any weaker than he already is. Never having had much luck with women has left him scared and scarred. He does not see or understand that Kate is hoping for more than brotherly love, but accepts her affection, which helps him heal. She doesn't speak up, fearing that he won't come to her on his own free will if he does not first get a chance to make up for lost experiences with other women.
Diana calls to offer him a job in her theatre company, which performs a comedic farce in a theatre outside London several nights a week. He starts out operating the lights, but when a lead actor trips and falls, he takes over the role of The Inspector. Diana invites him to her house and initiates a sexual relationship, which Martin welcomes; for the first time ever, he feels loved and desired. Sadly, Diana doesn't always have time for him, due to her busy career.
When the commercial airs, he is invited to reprise the role in a brief appearance on the Graham McAfee show. Initially he declines, thinking that his forays into show business will make it impossible to return to the world of real business, where he feels he belongs. But the money is good and he does have a debt to pay. And Martin always pays his debts. It's what ruined his life.
Backstage at the show he befriends Kelly, a young girl with a disease that carries a social stigma. He teaches her to dance. As they tour the studio, Martin saves the life of a production assistant who has a severe peanut allergy. When an actor fails to appear, he is invited to be a guest on the show. He prefers to do this in character, as the enigmatic butler Carstairs. On air, prompted by Martin, Kelly tells of her disease and turns to her new friend for support when a fellow guest is rude. Then they dance, for which Martin is not prepared. The emotional performance charms and moves the nation and makes Carstairs a household name overnight.
After Melody assists Martin and Kate in an elaborate ruse to get rid of the unwanted media attention, she shows an interest in him. They begin a sexual relationship, even though Martin makes it quite clear he wants to end up with Diana. Melody even helps him to find a place of his own, as Martin is concerned he is being a burden to his sister.
When Martin discovers Diana is married and has no intention of entering into a serious relationship, he is shocked and resigns from the show. Pretty soon after, his world comes tumbling down: due to a misunderstanding Melody comes to hate him, he is evicted by his landlord, what little money he has made is seized by the Dutch government and then he has a terrible argument with Kate, which ends in her kicking him out. Wandering the streets, homeless and penniless, he steps into a bar.
The next morning he wakes up in a police cell. Once released, he decides he will leave the country by signing up for a tough job drilling for shale oil in the United States. His final appearance in the papers is a summary of a drunk speech he gave the night before, taking out his anger on everyone he feels has wronged him.
In the nick of time, before he can disappear to America, Kate finds him in the London Underground and, fearing he might run away again, confesses her love. Even though she is his sister, she wants all of him. Martin has never loved any woman more than Kate and sees his sister in a new light. Loving her fully comes easy to him. After all, she has always been the love of his life. They kiss.