Warning: spoilers for Unforgotten series six episode one.Every series, ITV crime drama Unforgotten welcomes a new guest cast around whom that years investigation revolves. By the time the credits roll on the finale, series regulars DI Khan (Sanjeev Bhaskar) and DCI James (Sinad Keenan, who took over from Nicola Walkers DCI Stuart in series five) have uncovered exactly what role each played in that series murder, and then we never see any of them again until now.The final scene of Unforgotten series six, episode one featured the surprise return of a guest character from series four. Disgraced police detective DCI Ramjeet Sidhu, played by Phaldut Sharma (EastEnders, Hanna, Sherwood) was seen lying on his bed in a prison cell at HM Ashbourne. As Sunny explained to Jess, before Sidhu was locked up on multiple accounts of corruption in 2022, hed been the officer in charge of an investigation into the disappearance of Gerard Cooper, the man whose body parts have just surfaced in a London marsh.Despite the absence of a body, Sidhu had signed off on that investigations conclusion that Cooper had taken his own life by jumping into the Thames. Judging by the fact that the most likely cause of Coopers death was a stab wound to his femoral artery, and that his body was found sawn up, wrapped in plastic and thrown into marshland, this was clearly no suicide.The question now is: was Sidhus investigation merely bad police work, or was it another example of his corruption? (Incidentally, Sidhu worked in Vice, not Missing Persons, so disappearances wouldnt usually have come under his remit.)Why is DCI Sidhu in Prison?In series four, Sunny and Cassies investigation found evidence that Vice copper Sidhu was guilty of taking bribes to facilitate drug-smuggling. He and a businessman friend with whom hed been part of a conspiracy to cover up a murder decades earlier, had run a cocaine-import side line in the early 2000s that helped Sidhu sustain his flash lifestyle. When they got the team back together and carried out a one-off drug smuggling op in 2022, they were discovered.The corruption charges werent the only thing on Sidhus rap sheet; he was also charged with preventing a lawful burial in the case of the series four murder (see below), and had also had complaints brought against him for the use of excessive force against a member of the public and a fellow officer, and in series four, was also facing an investigation for the sexual harassment of a police officer. As one former colleague said of Sidhu hed either go right to the top or end up in prison. Thanks to Sunny, it was the latter.What Role Did Sidhu Play in the Series 4 Murder?In March 1990, a young Sidhu and four other probationers (police officers-in-training) were driving home from celebrating their passing out ceremony, when they spotted a small-time drug dealer who had sexually assaulted one of them in a pub weeks earlier, and who had beat Sidhu savagely when hed intervened on the victims behalf. Drunkenly, Sidhu and Dean (Andy Nyman) chased the man and, unseen by Sidhu, Dean killed him using a fountain pen. Sidhu arrived on the scene and unsuccessfully tried to revive the man with CPR.The five probationers decided to cover up the murder and dispose of the body. Only two of them stayed in the police service, and when the body was discovered and investigated by Sunny and Cassie 30 years later, they all tried to suppress the truth. Sidhu said he only went along with the cover-up because he feared that the other four probationers who were all white would conspire to blame the murder on him due to racism.Sidhus Wife and ChildSidhus storyline in series four didnt just involve the murder conspiracy, but also his second wifes pregnancy. At risk of having a baby with Downs Syndrome, he and his wife struggled with the decision of whether or not to continue with the pregnancy. Sidhu first of all wanted to keep the baby, but reconsidered when he realised his criminal activities could be discovered and his wife would be left alone to cope if he were sent to prison.While Sidhu was being booked on corruption charges in the final episode of the series, we were shown his wife buying a Babygro, so she must have decided to continue her pregnancy. In the series six scene of Sidhu in prison, her photograph and that of a baby are seen on the wall by his bed.Join our mailing listGet the best of Den of Geek delivered right to your inbox!Revenge on Sunny?Perhaps its just set dressing, but the book Sidhu is reading in his series six appearance is M.J. Arlidges Eye for an Eye, a novel about criminals living under witness protection having their identities exposed by a vigilante. Is that title a clue that hes also out for revenge, perhaps against the officer who charged him Sunny? Sidhu had a hostile attitude towards Sunny even before the charges were brought against him, due to their differing approaches to being British South-Asian police officers. Sidhu was angry, fought violently back against racist aggression, and was a member of the NBPA (National Black Police Association), while Sunny was not a member and refused Sidhus pally shared-heritage-based advances.Unforgotten series six airs on Sundays at 9pm on ITV1 and all episodes are streaming now on ITVX.