In 1948, during the Huaihai Campaign of the Chinese Civil War, Captain Gu Zidi leads the 9th Company of the People's Liberation Army 139th Regiment to capture a town controlled by the National Revolutionary Army, during which they sustain heavy casualties. Angered by the death of his political commissar, Gu attempts to execute the NRA soldiers who surrendered. As a result, he is thrown in jail by his superior, Colonel Liu Zeshui, for three days, where he befriends Wang Jincun, an enlisted man and former school-teacher who was jailed for cowardice. Upon being released, Gu and his 46 surviving men are sent to defend a coal mine on the bank of the Wen River, which is near Tai'an, Shandong. They are ordered not to retreat until they hear a bugle call for assembly. With Liu's permission, Wang becomes the 9th Company's new political commissar. Almost immediately after fortifying their position, the 9th Company comes under heavy attack by NRA forces. Having fended off a wave of enemy infantry, and destroyed two enemy tanks, only a handful of soldiers are still alive. At this point, some of the soldiers claim to hear the bugle call in the distance. Gu, who was temporarily deafened by an explosion, is reluctant to believe them and commands that they fight to the death. The entire 9th Company is killed except Gu, who is knocked unconscious by a tank shell.
PLA forces eventually recapture the area and find Gu, unconscious and heavily wounded, and wearing an enemy uniform. At the military hospital, Gu tries to explain that he was a captain in the PLA, and that, after being knocked unconscious, he awoke, put on the enemy uniform in order to steal food from nearby, and then bombed an enemy fuel depot before falling unconscious again. However, the PLA went through a reorganization while Gu was in a coma, and hardly anyone remembers the existence of the 139th Regiment, whose members nearly all perished, so Gu is unable to verify his identity. He is scorned by the other patients who consider him a deserter. Gu volunteers to fight in the Korean War as an enlisted man, and during an artillery spotting mission, risks his life to save his commander, Lieutenant Zhao Erdou, from a landmine, in the process losing his right eye. The two of them become close friends, and Zhao supports Gu's attempts to have his contributions and those of the 9th Company recognized.
Gu returns to the Wen River battlefield and tries to locate the mine. He is disappointed to see that the mine has been reactivated and that the old entrance has been buried underneath piles of coal. He encounters Wang's widow and convinces her to marry Zhao. As time passes, Zhao manages to discover the location of the tomb of Liu, the 139th Regiment's leader. Gu visits the tomb and finds that the curator is Liu's assistant and bodyguard, who survived the war, though he lost one of his arms. The curator confirms that the bugle call was never sounded, as the 9th Company was deliberately sacrificed to buy time for the rest of the regiment. Gu flies into a rage and fights with the curator but later manages to calm down.
Gu starts camping at the mine and attempts to dig out his men's bodies with a shovel, despite protests from the miners. When Wang's remains are found a month later, the PLA sends an official notice to the local government to honour the 9th Company. However, Gu remains inconsolable because he cannot find the remaining bodies. At this point, he experiences a flashback which reveals that, as enemy forces closed in, he and Wang buried the bodies of the others deep inside the mine. While Gu went out to continue fighting, a mortally wounded Wang blew up the entrance to prevent the enemy from capturing the bodies, killing himself in the process.
Years later, the remains of the other soldiers are found during an excavation for an irrigation project. The PLA erects a monument near the site and conducts a military funeral with full honours for the 9th Company. Gu finally finds peace. The ending titles say that Gu died in 1987 at the age of 71. He was abandoned by his parents when he was three months old due to famine in his hometown, and was found by a shoemaker in a millet field. He was named "Gu Zidi", which means "millet field".Zhang Hanyu as Gu Zidi (谷子地), the commander of the 9th Company.
Deng Chao as Zhao Erdou (赵二斗), an artillery battalion commander who befriends Gu Zidi.
Yuan Wenkang as Wang Jincun (王金存), the political commissar in the 9th Company.
Tang Yan as Sun Guiqin (孙桂琴), Wang Jincun's widow who remarried Zhao Erdou.
Liao Fan as Jiao Dapeng (焦大鹏), the commander of the 1st Platoon in the 9th Company.
Wang Baoqiang as Jiang Maocai (姜茂财), a sniper in the 9th Company.
Hu Jun as Liu Zeshui (刘泽水), the commander of the 139th Regiment.
Ren Quan as the original political commissar in the 9th Company who was killed at the beginning of the film.
Li Naiwen as Lü Kuangou (吕宽沟), a soldier in the 9th Company.
Fu Heng as Luo Guangtian (罗广田), a soldier in the 9th Company.
Zhao Shaokang as Laociwei (老刺猬), a soldier in the 9th Company.
Hu Ming as Xiaoliangzi (小梁子), the bugler in the 139th Regiment.
The action and effects team from the 2004 Korean war film Taegukgi were employed to work on Assembly. Assembly is also among the first films produced in mainland China to portray the Chinese Civil War in a realistic style. The film is also adapted from the novel Guan Si (A Legal Case), which is based on a real-life account of a veteran army captain upholding his company's honour.
The film was a massive box office success, particularly in mainland China.
Perry Lam gave a mixed review of Assembly in the Hong Kong magazine Muse: "There is a huge discrepancy between the sophistication of the filmmakers in their knowledge and application of state-of-the-art techniques, and the naivety and bad faith they place in the value of unquestioning obedience to authority and sacrifice as the highest manifestation of patriotism."
Kozo, who reviewed the film at the Hong Kong Asian Film Festival 2007, felt that Assembly is a safe commercial movie that does not offend anybody: "In Assembly, war is never really portrayed as a "cause". The human element is the main focus here, and the sacrifices made by soldiers are to be honoured because they're people, and not members of one side or the other."
Assembly was quickly followed by a sequel, Assembly 2: The Cold Flame (集结号2-烽火), which was also directed by Feng Xiaogang. Although it also featured Zhang Hanyu in a leading role, it was actually shot in 2005 and held back by the studio. It was eventually released to capitalise on the success of Assembly. The sequel centres on the relationship between a wounded NRA soldier and an orphaned girl during the Second Sino-Japanese War instead of the Chinese Civil War. It contains very few war scenes and focuses more on the personal drama between the characters.