I've been trying to find a specific story for a while now, but I'm having no luck. Though, luckily, I do remember quite a bit from from it.
Danny starts dating this girl (GF=girlfriend), who might be a nurse, and things move very fast over the course of weeks/months. She pretty much falls in love with Danny immediately, and Danny not so much (as time goes by). He finds her to be very clingy and odd. She keeps insisting on buying him really expensive things and is already taking about marriage, etc. She cooks for him all the time and so on. Eventually, Danny decides to break things off with this girl because he doesn't feel like he's in love with her (or like the relationship is going anywhere). This unleashes the GF's crazy/psychotic side.
Grace has a really bad relationship with this GF (who threatens to either send Grace away or take Danny away from her). Grace ends up hiding her feelings for Danny's girlfriend because she doesn't want to upset him and be sent away. I think the GF might have also been physically abusing Grace, and leaving her with bruises? Towards the end of the fic, Grace is pretty emotionally beaten and terrified of losing Danny.
After Danny breaks up with the GF, he goes back to her house (with Grace) because he thought he left his wallet. In reality, the girlfriend took it so Danny would have to come back to get it. Danny explains to Grace in the car outside of the house that this is the last time they have to talk with the GF and then their lives will be back to normal. When he arrives, the GF is fixing the sink in the kitchen, which she purposefully broke. It's creating a mess and she asks for his help. When he does, the GF attacks him with a knife and stabs him.
He ends up hiding with Grace in the bathroom. Meanwhile, the GF sets the house on fire because "if she can't have him, no one can". Danny forces Grace to escape the house through the tiny bathroom window and call for Five-0/their ohana for help on a cell phone a couple of blocks away. She ends up waiting for them there and to direct them to the house.
Steve busts onto the scene and calms Grace down. When he gets in the house, he ends up killing the GF while Chin calls for backup. Steve tries to open the bathroom, but it's locked/jammed. Danny is unconscious from blood loss so Kono climbs back through the window to help unlock the door to the bathroom to help free Danny. Then they all head the hospital?
I'm not sure where I read it since I've been trailing LJ, Ao3, and ff.net for H50 fics.