Nikki and the Lyell unit are called in to investigate an ysgou.cc attack against a transgender man.

　　Thomas and Jack are surprised to see Nikki back at The Lyell, the disappearance of her friend Sally forces her to face her back after her harrowing experience in Mexico.

　　This year Silent Witness celebrates its 20th anniversary with five explosive stories that take the show into new and uncharted territories as well as delving into the murky depths of city life. Five contemporary thrillers that start amongst the familiar landscapes of London and end up in the dusty scrubland of Mexico.

　　As Leo's replacement Thomas Chamberlain arrives Nikki investigates the murders of Rachel Freedman, shot in the head, and her little autistic son Nathan, who was poisoned. Nikki rejects policewoman Anne Burchett's theory that Rachel killed them both but is perturbed when an impostor, claiming to be Rachel's sister, visits the morgue. At the same time the disfigured corpse of Eva Liron is discovered, killed with the same poison as Nathan. She had been the nanny to French hot shot footballer Isaac Dreyfus and was planning to blackmail him over a sex tape of them both. Jewish Isaac is a controversial figure due to his anti-Muslim sentiments but he denies ant part in the murders. Then another victim, Isaac's agent Mousa, is discovered dead.

　　Following a court appearance for drunk driving, Leo is required to perform unpaid work as a punishment, assisting AJ, a community worker on a run-down,largely black-occupied housing estate in South London. AJ is hoping to divert boys away from crime by encouraging them to do sports, and, though initially dismissive of Leo as a white intruder, comes to respect him after he has saved the life of Errol, a former gang member now anxious to keep his little brother Levi out of trouble. Errol has been stabbed in the leg by members of a gang from a rival estate and Leo stops him from bleeding to death. Earlier in the week a young girl called Shana Block was stabbed to death at a funfair and her murder recorded on a mobile phone. As Shana lived on the estate police sergeant Wallace involves Leo as a go-between with the suspicious residents. Also on the scene are Harry and Nikki, the latter dating Ryan, a young Scottish paramedic she met when called to the scene of Shana's death.

　　The pathologists investigate when a military helicopter crashes into a refugee detention center. Harry and Nikki actually witnessed the crash and were first on the scene but the case is complex since the center was Ministry of Defense property leased to the Home Office. The team is shocked at the conditions they find in the center and Harry, who has a close friend in the RAF, is shaken when a government Minister attributes the accident to pilot error.

　　When a boat carrying illegal passengers is found crashed in the Thames, the team investigate.

　　The team is shocked when Leo Dalton's wife and daughter are killed when a fancy sports car careens out of control and smashes into a café. The police are puzzled by the cause of the crash but believe the driver may have been involved in a identity theft ring when they find several several passports and ID cards in the vehicle. Meanwhile, Nikki must perform urgent autopsies on two bodies found floating in the river.

　　A Time to Heal -The team is called out to Northern Ireland to investigate the bodies of two men killed by paramilitaries twenty years ago. For Sam, it is a case that is soon to turn personal…
　　Death by Water - Tensions are running high in the ‘leaderless’ team as Leo and Harry clash over an investigation into an outbreak of breathing difficulties among children on the South Coast.
　　Nowhere Fast - When members of a race horse owning syndicate are killed in a helicopter crash, Leo and Harry investigate – and an unexpected new face at the mortuary lends a helping hand.
　　Body 21 - Approached by a group of survivors from a recent train crash, the team investigate the wreckage and a new mystery – the identity of the unidentified body 21.

　　本劇改編自豆瓣閱讀連載小說《一生懸命》，作者、編劇陸春吾。
　　倪向東（尹昉 飾）收到好友曹小軍（白宇帆 飾）的請求，將一只木箱運送到荒山上的廢棄小屋，卻意外發(fā)現(xiàn)箱里裝的竟是曹小軍的尸體。曹小軍的妻子吳細妹（李庚希 飾）報警尋找失蹤的丈夫，并暗指一切都和倪向東有關(guān)。可作為證人，吳細妹的供詞前后矛盾，且夫妻二人的孩子并非曹小軍親生。在警方的連環(huán)追問下，吳細妹迫不得已講述了三人的往事。
　　三人曾是好友，吳細妹與倪向東是一對戀人，倪向東不愿為孩子負責(zé)，曹小軍挺身而出帶她遠走高飛，一起撫養(yǎng)孩子。在追查過程中，警方進一步發(fā)現(xiàn)吳細妹的孩子和倪向東也沒有血緣關(guān)系，大家對倪向東的評價還存在完全不同的兩種聲音：老家的人說倪向東無惡不作；工友卻說他與人為善，還資助了一位姓徐的老人。
　　老警員孟朝（張哲華 飾）帶著新人童浩（徐奧俊 飾），從北方海岸到南方密林，抽絲剝繭，揭開了吳細妹、曹小軍、倪向東糾纏了十余年的連環(huán)殺局，讓作惡之人得到了法律的制裁。