TBBC opens with the main character, Sehun's little sister Oh Semi, recovering from an assassination attempt in the United States.  The attempted killing is because she saw Luhan, a top mafia boss in Seoul, killing somebody, and she recorded this on her phone.  Since she's the only person with enough evidence prepared to testify against him, as well as to identify him, should the police be able to catch him, Luhan wants her eradicated and the phone taken care of.  It's also not been the first attempt on her life over the past year, and her bodyguard Kris (cameo role) is unsure whether or not it's possible to protect her anymore.

This leads to the case being referred back to the police department in Seoul, where Chen and Lay, the officers assigned to the case, are wondering exactly what to do about it.  Lay jokingly suggests playing the criminals at their own game by hauling Luhan's Blood Brother Xiumin out of prison and invoking part of the Blood Brother Code, which ensures the safety of family members of Blood Brothers, to keep Semi safe by her marrying Xiumin.  If, rather than hiding Semi (which appears not to be working) she's in plain sight, there's also the added bonus that Luhan might eventually show himself while trying to get her, and this could lead to his arrest.  However, Siwon (chief of police) overhears, thinks it's a fabulous idea, and decides that they'd better run with it.

Xiumin's not happy about agreeing, but reluctantly does when Chen and Lay threaten to have his case reviewed for the death penalty.  Once the plan has been passed by a fuming Sehun, Semi and Xiumin are married and given a car (bugged, wired, and potentially with explosives on board) and an apartment (also bugged and wired) that is under 24/7 surveillance for Semi's safety, as one of Xiumin's stipulations for agreeing to take part was that there wouldn't be bodyguards on top of him the entire time.  Computer technician extraodinaire Do Kyungsoo is responsible for this and also keeping out Luhan's computer hacker whizzkid (Infinite's Sungjong, though he is never named and never appears in person).  Kim Jongin, Kai's alias (he works as an agent and was one of Semi's bodyguards in the States), shares an office with Kyungsoo and is also often behind the surveillance cameras, while at the same time working under the cover of an incredibly goofy intern in order to flush out the spies that various gangs have among the police.

On the first night in the apartment, Xiumin scares Semi by reminding her that he used to run a ion ring, but assures her that she's not his type and that he's not interested in touching her - though he does question whether she is actually protected under the Blood Brother Code if they don't consummate the marriage.  He finds himself a separate room to sleep in, but Semi is sufficiently freaked out to lock herself into her en suite bathroom for the night.

Semi enrols herself in the nearby university, though it takes time for this to register, so she spends as much time out of the apartment as she can.  Xiumin goes round to the local restaurant, apparently on the pretext of eating high quality food after being in prison for two years.  Kyungsoo and Jongin follow, hoping that he'll leave clues that will lead them to Luhan, but nothing comes up - until it turns out that he's also asked one of the restaurant employees to give him a really old phone, which he proceeds to use on the apartment balcony to get in contact with Luhan.  He explains the situation to Luhan and asks for nine weeks, by which point he hopes that Semi will trust him sufficiently for him to be able to hand her over to Luhan, and also give time for Luhan to prepare an avenue of escape for him (aside from the surveillance, Xiumin has also been microchipped so that the police know where he is at all times).  They agree on a drop point during Sunday Mass in one of the confessionals in Myeongdong Cathedral as a standard way of communication so that they won't have to run the risk of being caught by using phones.  Kyungsoo is unable to pick up on the conversation as Xiumin has made use of a weak spot in the apartment's bugs (which Kyungsoo had deliberately set as a trap that Xiumin manages to avoid).

Xiumin gives Semi a diamond ring to wear that might serve to protect her a little (as the marriage would look more legitimate), but she's reluctant to put it on.  At university, she quickly makes friends with Tao, who is studying on the same course as her, and his friends Wendy and Seulgi, who share some seminars and lectures.  The next day, Xiumin and Semi go round to dinner with the neighbours, Mr and Mrs Kang, who have a son, Taeyong, whom Xiumin gets on very well with.  Semi unfortunately has a very bad allergic reaction to traces of nuts in the food that they're served, and has to be taken to hospital.

Soon after, Chen enlists Xiumin to help them with some cases revolving around gang crime in the city, as despite two years in prison he still has a reasonable working knowledge of who is in involved in what and influential in which areas.  Semi takes the opportunity to visit her brother, returning late enough for Xiumin to worry about where she has been, and when he tries to press her for details, Chen distracts them both by cutting all the power in the apartment so that Semi won't reveal any sensitive information, such as the existence of her brother Sehun, who Luhan might use against them if he knows about him.  This results in Semi injuring herself in the dark, which leads Wendy to conclude that she is in an abusive relationship with her husband, especially since Sehun had made a special effort to visit Semi on campus as he couldn't at the apartment and she had also missed a day of university due to her nut allergy.  Xiumin discovers Wendy's suspicions from Semi shortly afterwards and is perturbed, but following another misunderstanding between the pair of them, they sit down to figure out a fake story of how they fell in love, and also to draw up small character charts of each other so that they are both aware of essential information on the other person.

In dance class, Semi's dance partner Junhong manages to trigger a panic attack in Semi by accident, further fuelling Wendy's beliefs about abuse, but Semi insists that she's okay and makes friends with Junhong, and not long after they end up going to a small dance club on the other side of town.  Xiumin, meanwhile, pushes off to church, but keeps up pretences around the house by reading the Bible and asking Semi if she wants to go with him.  Semi also splashes out on buying a piano, which impressed Xiumin greatly as he never had the opportunity to play and appreciates her musical talent.  A day or two later, the pair of them end up babysitting Taeyong as his parents are away.  While the boy is there, Semi's PTSD from a major car crash which killed her father resurfaces in the form of nightmares, which concerns Xiumin and leads Sehun to tell her that she ought to consider getting treated again, especially since his housemate Chanyeol is a trained psychotherapist.  Xiumin, meanwhile, puts two and two together from Semi's phobia of cars and the trauma she is displaying, and figures out that she must have been in a nasty crash during wintry weather, as this also concerns her.  He tries asking her for details, but she is unforthcoming.

The nightmares persist for some time, also waking Xiumin up on a regular basis, and he starts leaving soup and either a drink or sleeping pills out for her to take as he views it would be healthier for her than brewing the copious amounts of coffee she tends to take to keep herself awake so that the nightmares won't return.  Things take a very slight turn for the better, and she agrees to go out clubbing with Tao, Wendy and Seulgi - which ultimately ends in disaster when somebody's inappropriate advances trigger a very nasty panic attack.  Xiumin arrives on the scene to help her calm down, but the nightmares become more frequent and more vivid.  Eventually, they become bad enough for her to seek out comfort from Xiumin (normally when they're very bad, she would seek out comfort from her brother, but under these circumstances, she can't) and spends the night in his bed.  This happens a couple more times, especially since the frosty weather triggers the memories more, until Xiumin points out that it sends off all the wrong kinds of signals.  Since she's distressed, he agrees to stay with her until she falls asleep, but it doesn't help very much.

Jongin, meanwhile, has managed to find himself a sweetheart in Song Qian, who also joins the group of police working on Semi's case.  Xiumin figures out that it's physical contact that helps Semi to keep calm and sleep well during the night and volunteers himself to help her, which makes a considerable difference.

When Semi next meets Junhong for dance, he's acting shifty.  They're cornered by some gangsters as they leave the building, but fortunately rescued by Xiumin and the