In "Run Silent, Run Deep," Stella found a website where Frankie had been posting video of them having sex. She confronted him about it and broke up with him. We find out about it in the episode you're talking about, "All Access." He follows her and even makes a clay mold of her key so that he can break into her apartment. When she comes home and tries to call someone to get him out of there, he attacks her, ties her up, drags her around the apartment, etc. When the doorbell rings for the take-out that Frankie had ordered, he leaves Stella in the bathtub, tied up. She broke a razor in the tub and used the blade to cut through the cords binding her. Then she hid behind the door and, when he came in and saw her gone, hit him with the door and ran out of the room, trying to go for her gun (I think, maybe it was her phone?). There was a bit of a scuffle and he got her gun, which he pointed at her and fired, but because he'd forgotten to take off the safety or something (nope, I have no idea what I'm talking about), it didn't fire. She grabbed it from him and shot him three times before collapsing onto the floor. A neighbor had heard the shots and called the cops. When Mac heard the address over the radio, he and Flack, along with some other cops, hurried over and broke in, where they found Stella unconscious and Frankie dead. The first we see of it is her passed out and Frakie's body, and the rest of the episode is about them piecing together what happened.
Stella is a CSI and is basically second-in-command to Mac. I suppose it was her day off, though.
Hope that helped.
