* CompleteMonster: In their reinvestigations UCOS have faced a variety of criminals, but these stand out:
** [[SmugSnake Ricky Hanson]], the series' only recurring antagonist and the personal ArchEnemy of [[TheMentor Jack]] [[TheLancer Halford]], was a mid-level LondonGangster described by Jack as "the biggest thieving, murdering, lying piece of shyte I've ever laid eyes upon." Starting out in the 1980s, Hanson took over the "[[ThoseWackyNazis Twenty-Fours]]" to use them as a front for his drug ring. When Ray Harris tipped him off that two members were really student activists, Hanson personally murdered them, then had their bodies hidden under Harris's conservatory, [[UngratefulBastard incriminating him]]. By 1996, Jack and Sandra Pullman were investigating Hanson for three murders, including [[CainAndAbel his own brother]]. Realising they were getting too close, Hanson ran over Jack's wife [[TheLostLenore Mary]], laughing as he did; it took two years for Mary's wounds to kill her. When his son, Luke, was wrongly accused of arson to save himself, Ricky left Luke to be sentenced to nine years in prison, and also stole Luke's girlfriend Lysette Perry. In the season 3 finale "Congratulations", Ricky confessed to Jack he killed Mary, tormenting him with the details of the event. Losing it, Jack attempted to kill Ricky; failing in the season 4 premier "Casualty", Ricky tried to slowly run him over but was forced to flee. Sneaking into Jack's hospital, Hanson assaulted the nurse then almost [[SickbedSlaying smothered Jack]]. Interrupted by Brian Lane, Hanson tried to strangle him. In season 6's "The Last Laugh", when the reinvestigation into the student's disappearance led back to him, Hanson set his goons on Gerry's informant Polish George. When their uncharacteristic mercy tipped Gerry that Polish had dirt on Hanson, Polish let slip that Lysette is Hanson's daughter. Learning that his father never told him he was with his half-sister broke Hanson's hold over Luke, leading to him finally being put away. Arrogant, coarse and brutish, Hanson was nothing more than a cruel thug, if an intelligent one.
** [[CorruptCorporateExecutive Alex Close]], from season 7's "Dark Chocolate", was an immoral small-time businessman, whose companies operated through the exploitation and overwork of illegal immigrants. Close proved to be an even more vile man, as he was also a SerialRapist who used his business's contracts to find victims. In 1992, he raped Jean Saunders at the chocolate factory where she worked, and then a few months later raped Eileen Harrison, permanently traumatising her. Close got around the mass DNA screening by forcing someone else to go in his place. When in 2001 his abuse of illegal immigrants was exposed, Close fled the country to Poland. Returning nine years later, he started a new business, this time using Polish immigrants, and immediately resumed his spree by attempting to get a bakery contract so he could rape his next target. When he failed, the undeterred Close resorted to regularly going as a customer, until he knew the layout well enough to rape Helen Vestry.
** [[HeroismAddict George Mackie]], from season 7's "Where There's Smoke", was a talented fire investigator respected for his [[ConsummateProfessional dedication and knowledge]]. However, he was secretly also a serial arsonist, later claiming to having started sixty-four separate fires. He would place homemade firebombs in his targets, then after the fire burned out, he would turn up to analyze it and [[GloryHound bask in the praise he received for his "insight"]]. In 1996, one of Mackie's bombs caused the Union club to burn to the ground, killing four people, including the club's owner, and causing seven hospitalizations. Shortly afterwards, Mackie retired and thus the fires stopped. UCOS's reinvestigation of the Union fire inadvertently inspired Mackie to come out of retirement. Breaking into David Swallow's house, he set up another bomb; Swallow was himself a survivor of the Union Club fire who had been left with third-degree burns over half his body. Mackie then set another bomb in Stuart Russel's house, this one nearly killing Jack, Brian and Gerry along with Russel. [[BitchInSheepsClothing Underneath all his charm and seeming helpfulness]], at heart Mackie was nothing more than a shameless AttentionWhore, happy to commit wanton destruction and kill innocent people just to [[ItsAllAboutMe stroke his own ego]].
** [[WolfInSheepsClothing Annabel Skinner]], from season 10's "The Little Brother", was a former associate of the notorious [[ConMan Larry Goftmon]]. To steal Florence Epstein's money, [[TheVamp she seduced Florence's husband Jonathan]] until he was so under her spell he agreed to murder Florence and steal her money. They [[ColdBloodedTorture tortured her bank details out of her]], before bludgeoning Florence to death with a [[DropTheHammer hammer]]. No longer needing him, Annabel turned Jonathan over to the police. However, Goftmon double-crossed her and ran off with the money. Desiring vengeance, not just against Goftmon but also the accountant Peter Sale--[[MisplacedRetribution despite him being unaware of their plan and being cheated himself]]--she [[NeckSnap violently murdered]] Sale's sister Margaret Kirby and [[DeadPersonImpersonation stole her identity]]. After six years of searching she found Goftmon, but couldn't find Sale. Devising a plan, she set up a trail directing towards him and got acquainted with Esther Lane. [[BitchInSheepsClothing Playing the role of a caring sister who wanted to tell her brother he was an uncle]], she thus sent Brian on the trail to find Sale. Skinner then tipped the police about Gofton's new alias, before strangling him and leaving the body for Brian to find. Finally finding Sale, Skinner disabled the police officer who was guarding him and tried to murder him. When Brian interrupted her, she tried to strangle him too. A manipulative violent [[TheSociopath sociopath]], Skinner even described herself as "[[CardCarryingVillain fundamentally nasty]]".
* EarWorm: "It's all right, it's okay"
* HilariousInHindsight: In one early episode Gerry is told by a scrap metal merchant that his car, a classic Triumph Stag, is barely even worth scrap metal prices because no one buys them. The joke obviously being on Gerry, since he is only there on enquiries about the case, for driving a rubbish car. Since that episode was made, classic car prices have sky-rocketed and Triumph Stags have become retro-cool with even bad examples making serious money (this is partly due to the discovery that the "problems" Stag engines had was actually due to their radiators not being flushed properly by owners as part of the maintenance regime).
* ItWillNeverCatchOn: TV critic AA Gill has been widely mocked for claiming the show would sink without trace when it first debuted; of course it has not only been going for more than a decade with very high viewing figures, but is also one of the most popular British export programmes.
* MagnificentBastard: MI5 agent Stephen Fisher (played by Tim [=McInnerny=]) in Series 9.
* SuspiciouslySimilarSubstitute: Averted with Steve replacing Jack, as they are very different characters. Less averted for Dan replacing Brian, as they are both TheSpock and have family issues (though different ones) and for Sasha replacing Sandra.
* TheyChangedItNowItSucks: The reason Waterman left the show - when all the other members of the original cast had left the show Waterman went on record to say that he missed the chemistry he had with them and it's just not the same without them so he decided to move on too.