The Big Heat

Homicide detective Sergeant Dave Bannion (Glenn Ford) investigates the suicide of officer Tom Duncan, which seems to have been brought on by ill health. Bannion is contacted by the late cop's mistress, Lucy Chapman (Dorothy Green), who claims it could not have been suicide. Bannion revisits his widow, Bertha Duncan (Jeanette Nolan), asking for particulars on the second home, but she resents the implication. The next day, Bannion is rebuked by Lieutenant Ted Wilks (Willis Bouchey), who is under pressure from "upstairs" to close the case. Chapman is found dead after being tortured and covered with cigarette burns. Bannion investigates, although the case is not in his jurisdiction. After receiving threatening calls to his home, he confronts Mike Lagana (Alexander Scourby), the local mob boss who runs the city. and finds that people are too scared to stand up to the crime syndicate. When warnings to Bannion go unheeded, his car is blown up and his wife Katie (Jocelyn Brando) is killed. After accusing his superiors of corruption, Bannion resigns.
When Lagana's second-in-command Vince Stone (Lee Marvin) punishes a girl in a nightclubâby burning her hand with a cigar buttâBannion stands up to him, which impresses Stone's girlfriend Debby Marsh (Gloria Grahame). Marsh tries to get friendly with Bannion, who keeps pointing out that she gets her money from a thief. When Debby unwittingly reminds Bannion of his late wife, he sends her out of his hotel room. Debby had been seen with Bannion and when she returns to Stone's penthouse, he accuses her of talking to Bannion about his activities and throws boiling coffee in her face. Debby is taken to hospital by Police Commissioner Higgins, who was playing poker with Stone and his group at the flat. With her face disfigured, Debby returns to Bannion, who finds her a room at his hotel. Debby identifies the man who had arranged the planting of the dynamite as Larry Gordon (Adam Williams), one of Stone's associates. Bannion forces Gordon to admit to the bombing, who reveals that Duncan's widow has papers that could expose Stone and Lagana, and is collecting blackmail payments from Lagana. Bannion refrains from killing Gordon, instead spreading the word that Gordon had talked, and Gordon is murdered by Stone's men. Bannion then confronts Mrs. Duncan, accusing her of betraying Chapman, causing her death and protecting Lagana and Stone. Cops sent by Lagana arrive before he can strangle her and Bannion departs.
Stone decides to kidnap Bannion's young daughter Joyce (Linda Bennett), who is staying with an aunt and uncle under a police guard. When the police guard is called away at the behest of Lagana, the uncle calls in a few army buddies for their protection. Bannion then sets off to deal with Stone, meeting Lieutenant Wilks (Willis Bouchey), who is now prepared to make a stand against the mob. Debby goes to see Mrs. Duncan, noting they are both wearing the same expensive coats and have benefited from an association with gangsters, and kills her. Stone returns to his penthouse and Debby throws boiling coffee at him. Stone shoots her but after a short gun battle with Bannion, who had followed him, is captured. As Debby lies dying, Bannion describes his late wife to her in terms of their relationship, rather than the physical "police description" he gave earlier and tells her that she and his wife would have gotten along. Stone is then arrested for murder, Duncan's evidence is made public, and Lagana and Commissioner Higgins are indicted. Bannion returns to his job at Homicide.

Question: Who is late cop's mistress?
Answer: Lucy Chapman