Basic Instinct

A retired rock star, Johnny Boz, is stabbed to death with an ice pick during sex by a mysterious blonde woman at his apartment. Homicide detective Nick Curran investigates, and the only suspect is Catherine Tramell, Boz's bisexual girlfriend and a crime novelist who has written a novel that mirrors the crime. It is concluded that either Catherine herself did it or someone trying to frame her out of spite. Tramell is uncooperative and taunting in the investigation, smoking in the interrogation room and exposing her bare genitalia in front of the officers. She presents alibis and passes a lie detector test. Nick discovers that Catherine has a habit of befriending murderers, including her girlfriend Roxy, who is later shown to have murdered several young boys on impulse, and Hazel Dobkins, who murdered her family.
Nick, who accidentally shot two tourists while high on cocaine, attends counseling sessions with police psychologist Dr. Beth Garner, with whom he has had an affair. Nick discovers that Catherine plans on using him as a fictional detective in her latest book, wherein his character is murdered after falling for the wrong woman. Catherine becomes aware of Nick's past after paying Lt. Nielsen to look into Nick's psychiatric file; Beth gives it to him after Nielsen recommends Nick's termination. Nick publicly assaults Nielsen in his office and later becomes a prime suspect after Nielsen is killed. Nick suspects Catherine, and when he joins in her behavior in front of his co-workers, he is put on leave.
A torrid affair between Nick and Catherine begins with the air of a cat-and-mouse game. Nick shows up at a club and witness her sniffing coke in a bathroom stall along with Roxy and another man. Nick and Catherine begin to dance and make out at a club. Later, observed by Roxy, they have sex in a bed. Roxy, jealous of Nick, attempts to run him over with Catherine's car, but dies in a crash when the car goes off the edge of the road. Catherine is saddened by Roxy's death and reveals to Nick that a previous lesbian encounter at college went away when the girl, Lisa Hoberman, became obsessed with her, causing him to believe that she may not have killed Boz. Nick identifies the girl as Beth Garner, who acknowledges the encounter, but claims Catherine was the one who became obsessed.
Nick discovers the final pages of Catherine's new book in which the fictional detective finds his partner lying dead with his legs protruding between the doors of an elevator. Catherine breaks off their affair; Nick becomes upset and suspicious. Nick later meets his partner Gus, who has arranged to meet with Catherine's college roommate at an office building to find out what really went on between Catherine and Beth. As Nick waits in the car, Gus is stabbed to death with an ice pick. Nick runs into the building, but is too late; he finds Gus' legs protruding from the doors of the elevator. Beth, standing in the hallway, explains she received a message to meet Gus. Nick suspects she murdered Gus and when he believes she is reaching for a gun, he shoots her only to find that Beth was only fingering an ornament on her key chain.
A search of the scene and Beth's apartment turns up the evidence needed to identify her as the killer. Despite knowing Catherine's foreknowledge of Gus' death, that she must actually have been the killer, and that she must have set up Beth, Nick tells no one. He returns to his apartment where Catherine meets him. She explains her reluctance to commit to him and the two have sex. As they discuss their future, an ice pick is revealed to be under the bed.

Question: Who does Nick have an affair with?
Answer: Dr. Beth Garner