Ruthless People

Millionaire Sam Stone (Danny DeVito) hates wife Barbara (Bette Midler). He plans on murdering her to gain control of her $15 million family fortune and run off with his mistress Carol (Anita Morris). He goes home to murder his wife, but he he can't find her. The phone rings. He answers. An anonymous man tells him that Barbara has been kidnapped and if Sam informs the media, the police or any of their detailed demands are not met - that they will kill his wife.
Overjoyed, Sam informs the media. the police and deliberately disobeys all of the kidnapper's demands, believing this will ensure his wife's death. However...
The kidnappers are Ken (Judge Reinhold) and Sandy Kessler (Helen Slater), who want revenge on Sam for stealing Sandy's fashion design, along with the Kesslers' life savings. Barbara is imprisoned in the Kesslers' basement, where she proves a handful to the amateur kidnappers. When Sam doesn't show up with the ransom on several occasions, even when the price is dropped, it becomes obvious that Sam doesn't want his wife back, and would rather she were dead.
Carol, having learned of Sam's plan to kill Barbara, secretly intends to blackmail Sam, with the help of her handsome but dim-witted boyfriend Earl (Bill Pullman). Knowing Sam plans to dump his wife's body in the Hollywood Hills at night, Carol has Earl lie in wait with a video camera. He mistakenly films a rendezvous between a prostitute and her client performing noisy sex in the front seat of a car. Earl, hearing the woman's screams, thinks the murder is happening right in front of him.
Without watching the tape, Carol sends an anonymous copy to Sam, who sees the sex act and thinks Carol has sent it to him as a tittilating birthday present. He tells Carol he will do the same thing to her, causing her to think he plans to kill her. Carol sends another anonymous copy to police chief Henry Benton (William G. Schilling) â who happens to be the prostitute's client. Benton, thinking that he is being blackmailed, asks for the demands. Carol tells him to arrest Sam Stone for murdering his wife.
Benton orders a search of Sam's house, planning to plant evidence, but real evidence turns upâa bottle of chloroform Sam intended to use to sedate his wife. There also are pictures of Sam with Carol. The kidnapping investigation, which has led to Ken by now, is immediately called off, and Sam is arrested. Sam now faces the unhappy prospect of having to get his wife back in order to prove his innocence.
While being held captive in the Kessler's basement, Barbara takes up exercising to relieve her boredom. Sandy half-mentions that she looks great and that it looks like Barbara has lost at least 20 pounds. Unexpectedly, Barbara bonds with Sandy, letting Barbara wear some of her dress designs to show off her new figure. Barbara loves them, and offers to go into business with Sandy. During their bonding, Sandy reveals why they originally kidnapped her, because Sam said she was his partner when he ripped them off. Barbara laughs and says she never was and Sam was mostly likely "passing the buck". Much to her shock, Sandy tells her Sam wouldn't pay their ransom demand. Even though it wasn't a big problem, she tells Barbara that Sam complained and they even tried lowering the price. She cries, stating "I've been kidnapped by K-Mart!"
Permitted to leave, Barbara later returns to the Kessler residence as soon as she finds out from the newspaper about Sam's mistress; Barbara now realizes Sam wanted her dead. Unbeknownst to Barbara and the Kesslers, a notorious local serial killer, The Bedroom Killer, had just entered their home and confronts both Barbara and Ken since they reminded him of his much hated parents. The serial killer ends up attacking Ken, but instead falls down the basement steps and dies.
Barbara, Ken, and Sandy now set a revenge plot in motion against Sam. Desperate to prove his wife is alive, Sam offers to pay the ransom the moment Ken calls him again. Armed with Barbara's inside knowledge of Sam's finances, they have increased the ransom to equal Sam's entire net worth: over $2 million. Sam is outraged, but has no choice. He withdraws the cash, but begs the police to watch the drop-site. Carol finally views the videotape in a video store, but unknowingly puts the image on every TV in the store, and the police chief is recognized by his wife. Carol realizes now Barbara really was kidnapped. Carol learns the time and place of the ransom drop.
Sam waits with his life savings in cash in a briefcase. Ken arrives in disguise to get the money, but then scores of hidden police suddenly appear. Sam gives the briefcase to Ken, but Earl arrives with a gun, intent on robbing Sam. He instead tries to rob Ken (who is holding the briefcase). In the ensuing confusion, Earl is captured by the police.
Ken takes the briefcase and drives toward the waterfront, with many police cars following him. He drives onto â and eventually off of â the end of Santa Monica Pier with the ransom cash inside. The police search the water and bring up the car, with the body of the Bedroom Killer inside (dressed in Ken's clothes and disguise). Only a few thousand dollars of money are recovered from the ocean.
Although he has lost all his money, Sam holds out hope that Barbara will now definitely be killed. According to the kidnappers, if anything went wrong, they'd kill her, and he'd inherit her $15 million fortune. But Barbara shows up and lies to the police that her kidnapper (the serial killer) was schizophrenic, believing himself to have an accomplice, and so she was able to escape as soon as he left. The police walk away in satisfaction.
Sam, meanwhile, is taken aback by how great Barbara looks with her weight loss. As they embrace, she beats him up in retaliation for all he's done and didn't do during her kidnapping (while the detectives in charge of the case remark about how much Sam and Barbara seem to love each other, unaware of the assault right behind them), and pushes him into the water. On a nearby beach, Ken emerges from the water in scuba gear, carrying the briefcase with the ransom cash. Sandy runs to embrace him. They are joined by Barbara, walking along the beach happily together.

Question: Who is Carol?
Answer: Sam Stone's mistress.