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　　号称市局“三叉戟”的崔铁军、徐国柱和潘江海在侦破一起传销案时，发现关键嫌疑人和二十年前一桩凶案有牵连。为了能够彻查案件，三位老警察在郭局的支持下，成立了三叉戟工作室。在徒弟小吕和年轻女干警徐蔓的配合下，老三位协助各警种破获了多起大案、要案，那桩二十年前的疑案也最终真相大白。然而随着调查的深入，三叉戟发现这一切都和一个叫做“青山会”的犯罪集团有关。而这个犯罪集团试图通过“云贸币”这种虚拟货币，垄断市场攫取利益。最终，三叉戟迎难而上，突破重重阻力，终于将这个盘踞平安省多年的犯罪组织“青山会”连根拔除，主犯周茂被捉拿归案。三叉戟再一次保护了平安百姓的生命财产安全。

　　1942年，瓜达卡纳尔岛，美军陆战第1师占领了该岛上的日军机场，由此拉开了太平洋战争上的战略对日大反攻。三个美军海军陆战队士兵莱基、斯莱治和巴斯龙在太平洋上经历了二战时期最为惨烈的太平洋战争。在太平洋上，日本陆军骄横狂妄，对美军发起了猛烈的攻势，期间不乏使用自杀式的袭击。从惨烈的瓜达卡纳尔岛战役、格洛斯特角登陆战，及热带丛林战役，贝里琉岛争夺战，到硫磺岛战役直至冲绳岛战役。莱基（詹姆斯·贝吉·戴尔 James Badge Dale 饰）、斯莱治（约瑟夫·梅泽罗 Joseph Mazzello 饰）和巴斯龙（乔恩·塞达 Jon Seda 饰）见证了日军的负隅顽抗直至最后的溃败，也亲眼看到战友不断的在这惨烈的战斗中牺牲。作为最贵的二战美剧，HBO为此耗资2.23亿美元，这也是斯皮尔伯格与汤姆·汉克斯继《兄弟连》之后的再次联手。

　　The detectives investigate a multiple homicide at a fast food restaurant; an addict gives his baby daughter to a drug dealer as collateral on an overdue loan; Internal Affairs reopens the Kirkendall case.

　　In the four months since Joey Salvo's assassination, Simone is working odd jobs since he has been suspended by Internal Affairs pending their investigation, while he approaches Sipowicz asking about his and others incitements by the FBI who think that Salvo's death was an inside job within IAB, and Lt. Shannon from IAB tells Lt. Fancy that he may know the person who killed Salvo.

　　In the fourth season of this award-winning show, the action at New York City's 15th Precinct is as hectic as ever. While Lieutenant Fancy continues as precinct commander, Detective Andy Sipowicz, although still a bit rough around the edges, has mellowed with the birth of his son. Detective Bobby Simone and Detective Diane Russell take the next step in their relationship, James Martinez becomes the Squad Delegate, and Greg Meadavoy considers becoming a surrogate father. But one thing that hasn't changed is the squad's determination to make sure the bad guys get their day in court and to also attempt to maintain a touch of normalcy in their personal lives even as they investigate cases that often put them directly in the line of fire.

　　At the beginning of the season Sylvia becomes pregnant with Andy's child. A baby boy, Theo, is born towards the end of the season. This is contrasted with the fate that awaits Sipowicz's older son, Andy Jr., who announces that he is to join the police force. Andy is finally bonding with his estranged son when he is gunned down, which leads the elder Sipowicz to fall off the wagon. Andy Jr's murderers are killed themselves by Simone in an act of self defense.
　　Bobby and Diane, whose relationship had been put on hold while she attended AA, resume their relationship only for Diane to begin drinking again when her abusive father beats her mother. Her father is eventually killed and her mother becomes the prime suspect.
　　James Martinez and new detective Adrienne Lesniak begin an affair but only after Lesniak tells Medavoy that she is gay, then that she is not; Martinez later breaks up with her due to her controlling and unpleasant behavior, and Lesniak eventually leaves the squad. Medavoy himself leaves his wife, recognizing that she is holding him back but it is too late to save his relationship with Donna who leaves to take up a job with Apple.