* AlasPoorScrappy:
** [[spoiler:Maya]]. Fans didn't like her for being some random new love interest for Lucas who got way too much CharacterShilling, despite the fact the viewers didn't know a thing about her, which led to them questioning [[WhatDoesHeSeeInHer why Lucas really cared that much for her]]. Nevertheless, [[spoiler:Maya dying right beside him after he threw his entire life down the toilet just to be with her was pretty damn harsh]].
** No one really liked [[spoiler:Tariq Masood]] for being a boring ReplacementScrappy for Malcolm. But watching as he's [[spoiler:poisoned by some "vagrant" he bumps into, and then crying in the back of a taxi cab upon realizing this shortly before he collapses on the sidewalk and dies in front of Calum]] is a horrible and anticlimactic way to go. It certainly didn't help that he was ''finally'' starting to get CharacterDevelopment in the episode he died in.
* CompleteMonster:
** The first episode, "Looking After Our Own," has the far-right white supremacist Robert Osbourne. Planning a series of race riots to force Parliament to crack down on immigration, Osbourne also [[DomesticAbuser viciously abuses his wife Claire]]. When he realizes MI5 is watching him, he has their contact murdered. Osbourne is revealed to be connected to a human smuggling operation so he can choke off the asylum centers- with the immigrants occasionally thrown into the ocean. When he catches the agents after him, Osbourne shows what a monster he is by torturing Helen. First he forces her hand into a deep fat fryer. [[CruelAndUnusualDeath Then her head.]]
** Mohammed Rachid is a [[TheFundamentalist radical mullah]] who has [[TheUsurper taken over]] a Mosque in Birmingham England, from the kind and moderate Imam who founded the Mosque. Rachid hates all Westerners and uses the Mosque as a recruitment ground to convince disaffected Muslim teenagers to become [[SuicideAttack suicide bombers]]. When Rachid discovers one of his young followers is [[TheMole a mole]] for MI5, he has the young man beaten with sticks and thrown out a window. Rachid sees his suicide bombers as disposable [[ThePawn pawns]] in his mission against the West and has no intention of endangering his own life for that mission. Rachid's ultimate target for his suicide bombers is [[WouldHurtAChild a playground full of children]].
* HarsherInHindsight: During one of Lucas' flashbacks where he was being held in a Russian prison, [[spoiler:he tries to hang himself, presumably because he couldn't take the torture anymore]]. Skip to the penultimate episode of Series 9 and you'll discover the ''real'' reason why he [[spoiler:attempted to kill himself--and you may even hate the fact that he didn't succeed]].
* HilariousInHindsight: [[spoiler: Connie's [[RunningGag apparent inability to remember the current name of the Russian security service]] becomes ''really'' ironic when she turns out to be the Russian mole.]]
* IronWoobie: [[spoiler: Harry. Specifically; having friends and colleagues frequently die, repeatedly being rejected by Ruth, having certain close friends and trusted allies betray him (and in one case having to kill them - Series 9 episode 1). Yes, Peter Firth is the master of sad and tired expressions.]]
* ItWasHisSled: Helen gets her face dunked in a deep-fryer and then gets shot in the head early on in the show.
* MagnificentBitch: [[spoiler:Elena, who starts out being [[BitchInSheepsClothing a sweet and innocent long-lost lover for Harry]], but ends up being the BigBad for all of Series 10 [[ManipulativeBitch who used Harry and his team]] to her advantage.]]
* JumpingTheShark: Some fans felt that the show wasn't the same after [[spoiler:Adam died in the Series 7 premiere]]. Others would say it reached its peak after [[spoiler:the Series 8 finale where Ros died]].
* {{Narm}}:
** Paul Rhys' [[HamAndCheese incredibly hammy]] performance as Alexis Meynell in "On the Brink." Especially during his '''"WHERE ARE ALL YOUR BAAAAAWWWWWLSSS?!"''' speech, and the part where he randomly snarls and shakes a handrail upon discovering that [[spoiler:Ros works for MI5]].
** The sixth episode of Series 8 involves a banker targeted by a conspiracy, which [[spoiler: murders his girlfriend and her son when they break into their flat]]. The team keeps this secret from him until he learns the truth from a news report [[spoiler: and makes the most hilarious face ever seen on television]] which undermines an otherwise very dramatic scene.
** Ashur sobbing in episode 10.04 can come off as this since he sounded more like a dying animal moaning. Given the situation, it's NarmCharm for some.
* RescuedFromTheScrappyHeap:
** Ben Kaplan wasn't much of a likeable character for being a {{Jerkass}} and a cliché journalist who only cared about his personal career. Come Series 7, he TookALevelInBadass and has become more fleshed out, and he took his job as an MI5 agent more seriously.
** Dimitri wasn't well-liked for being a boring agent randomly thrown into the show for the sake of having more characters. But after his DayInTheLimelight in episode 10.03, fans got to know more about his personality, and his role in the show became much more important.
** Erin Watts, who was a ReplacementScrappy for BaseBreaker Beth. Like Dimitri, fans warmed up to her halfway into Series 10 when [[ADayInTheLimelight she had an episode that focused a lot on her]]. It certainly didn't hurt when it was revealed that she thinks people can work at MI5 and still have a family, or that she began to realize [[BeingGoodSucks how horrible it is to work as an agent]].
* TheScrappy:
** Maya, Lucas' love interest in Series 9. Hardly anyone likes her for being a FlatCharacter who got no CharacterDevelopment whatsoever. The fact that [[CharacterShilling Lucas continued to praise how wonderful and special she is]], despite the viewers knowing nothing about her personal history, was even more annoying.
** Calum is arguably the most hated MI5 officer in the whole show for being a [[DeadpanSnarker snide]] {{Jerkass}} who whines at times and thinks his sarcastic behavior [[DudeNotFunny is actually funny]]. While he did become a bit nicer after [[spoiler:Tariq's death]], his snarky attitude still remained.
* SeasonalRot: Very evident by series nine as the show drifted away from its commitment to showing the characters' personal lives and/or tied them into the plot at the expense of plausibility. Several replacements for dead LongRunners in the cast were felt to be [[FlatCharacter sorely underdeveloped compared to their predecessors]] because of this shift in focus.
* ShockingSwerve:
** As mentioned on the main page, [[spoiler:Juliet Shaw's FaceHeelTurn that put her in charge of series six's season-long antagonists, Yalta, who were staunchly anti-American to the point of supplying Iran with the components for nuclear weapons just to keep the US in check. Besides having been shown to have strong pro-American sympathies in series four, Shaw wasn't present in series six until the reveal in episode eight (having been CommutingOnABus with handicap access since the series five premiere), which prevented any meaningful build to the moment. Didn't help that her paralysis was also retconned in this episode]].
** Invoked in Series 10 with the discovery that [[spoiler:Sasha Gavrik is really Harry and Elena's son.]] It feels forced and seemingly out of place because it's ''supposed'' to be. [[spoiler:In the end, it's revealed that Sasha isn't Harry's son at all; Elena lied her ass off just to make Harry think he was the father for thirty years]]. ManipulativeBitch indeed.
* TheyWastedAPerfectlyGoodCharacter: Lucas North was introduced in the first episode of series 7 pretending to be a double agent for the Russian Federation with his Russian ex-wife Elizaveta as his handler. All in all, an interesting and shady change of pace from relative boy scout Adam Carter. An early episode even revolved around Lucas and Elizaveta being murdered as traitors by Russia if MI-5 acted on intel he got from the Federation. His precarious life as a triple agent seemed like it would be a recurring plotline for him. It lasted five episodes, not even to the end of series 7, before it was dropped for good and he became basically the new Adam Carter. At least until series 9 happened....