History YMMV / The39Clues

* SurprisinglyImprovedSequel: The third series, ''Unstoppable'', set to right a few wrongs from the second series, "Day of Doom":
** The EvilPlan of the Vespers ([[spoiler:Using a DoomsdayDevice to destroy the world]]) was deemed too cliché. This time, the main villain's plan is to [[spoiler:mass-reproduce the master serum for him and his closest allies, then become the United States President and start a new World War to garner support for him]]. This was seen as superior.
** [[KilledOffForReal The killing of minor and major characters]] ([[spoiler:William [=McIntyre=], Erasmus, Luna Amato, Alistair Oh, Natalie Kabra, Evan Tolliver, Isabel Kabra and Damien Vesper III]]) was criticized for being [[WhatDoYouMeanItsForKids too dark]], [[SacrificialLion cheap ways to raise the stakes]] and [[TheyWastedAPerfectlyGoodCharacter complete wastes of interesting characters]]. In this series, while there is only one minor character who dies ([[spoiler:Pony]]), no other significant characters bite the bullet by the end, which was praised, because the stakes were still high without needing to resort to pointless deaths.

* FridgeBrilliance: In the first series, Amy and Dan were often shown to have [[TwinTelepathy sibling telepathy]], but that has all but disappeared in the SequelSeries. It is because, as Dan grows darker and more emotionally distant from his sister, he and Amy can't understand each other as well as they used to.

* AuthorsSavingThrow: The third series, ''Unstoppable'', set to right a few wrongs from the second series, "Day of Doom":
** The EvilPlan of the Vespers ([[spoiler:Using a DoomsdayDevice to destroy the world]]) was deemed too cliché. This time, the main villain's plan is to [[spoiler:mass-reproduce the master serum for him and his closest allies, then become the United States President and start a new World War to garner support for him]]. This was seen as superior.
** [[KilledOffForReal The killing of minor and major characters]] ([[spoiler:William [=McIntyre=], Erasmus, Luna Amato, Alistair Oh, Natalie Kabra, Evan Tolliver, Isabel Kabra and Damien Vesper III]]) was criticized for being [[WhatDoYouMeanItsForKids too dark]], [[SacrificialLion cheap ways to raise the stakes]] and [[TheyWastedAPerfectlyGoodCharacter complete wastes of interesting characters]]. In this series, while there is only one minor character who dies ([[spoiler:Pony]]), no other significant characters bite the bullet by the end, which was praised, because the stakes were still high without needing to resort to pointless deaths.
** [[spoiler:When Dan failed to drink the master serum because of Amy and Isabel preventing him, it was deemed a [[TheyWastedAPerfectlyGoodPlot wasted opportunity]] to explore themes of absolute power's corruption and morality. This plot line returns with a vengeance, with Amy drinking the serum in "Countdown". As a result, her story onwards from that point was loved by fans, who thought her struggles with the physical and psychological effects of the master serum were compelling to read about]].
** [[spoiler:The [[SurprisinglyHappyEnding surprisingly]] [[EsotericHappyEnding happy ending]], with the world saved, and Amy and Dan happily heading off to resume their lives, was mocked due to its ignorance of all the heavy emotional scarring that the Vespers' presence would bring. The first book of this series, "Nowhere to Run", showed, right off the bat, that Dan and Amy are '''not''' okay with how the Vespers manipulated them, as they harbor PTSD over this. Not only that, they have been having tutors instead of school, just so they can feel safe]].
** The RomanticPlotTumor of [[spoiler:Amy and Jake]] getting together was savaged for the [[OutOfCharacterMoment OOC moments]] that it created for [[spoiler:Amy]], the [[spoiler:LoveConfession being written cheesily]], and the mere notion that [[spoiler:by "Day of Doom", Amy still wanted to continue her newly-budded relationship with Jake, despite her former boyfriend Evan dying just days ago]]. A subplot was created for this series, with [[spoiler:Amy repeatedly trying to force Jake away to keep him safe, and remain JustFriends, as she clearly still grieves for Evan]]. [[BrokenBase Whether this worked is up for debate]].

** Isabel Kabra crossed it when she blackmailed the other Cahills into giving her their clues by [[IHAveYourWife taking their loved ones hostage]]. She even [[AbusiveParent shot her own daughter in the foot]] and [[spoiler:as Vesper Two, was partially responsible for her abduction]].

** [[EvilOldFolks The Outcast]] not only attempted to recreate four historical disasters to challenge the Cahills; [[spoiler: he [[BeenThereShapedHistory deliberately caused the original]] UsefulNotes/{{Chernobyl}} disaster to cover up his own illegal activities, and was more than willing to do it again on a larger scale]].

** [[spoiler: Ian's famous "Shoot to kill?" line in ''The Medusa Plot'' is suddenly not so funny anymore after Evan is ''actually'' shot and killed at the end of the SequelSeries. After he and Ian seemed to have been starting to develop a friendship, no less.]]

** [[EvilOldFolks The Outcast]] not only attempted to recreate four historical disasters to challenge the Cahills; [[spoiler: he [[BeenThereShapedHistory deliberately caused the original]] UsefulNotes/{{Chernobyl}} disaster to cover up his own illegal activities]].

* TheyWastedAPerfectlyGoodCharacter: Let's just say that with the main cast being an extended family with LoadsAndLoadsOfCharacters, this was bound to happen:

* FoeYay: Seeing as most of the main characters are adolescents caught up in centuries-old feuds, this is eveywhere. Mostly between Amy and Ian, [[spoiler:especially in the third book]], but it can be seen between other clue hunters, too.

* FunnyAneurysmMoment:
** In ''One False Note'', while Nellie is waiting for Amy and Dan, who are pursuing their Korean cousin, Alistair Oh, she ponders that if Alistair would do anything horrible to them, she would "feed him" his own diamond-tipped cane, wrapped in barbed wire. When ''Comicbook/TheWalkingDead'' introduced a barbed wire-wrapped baseball bat-wielding villain named Negan, [[WhamEpisode he introduced himself as a leader not to be messed with]] [[YourHeadASplode by repeatedly bludgeoning one of the main protagonists to death]], a Korean named Glenn.
** [[spoiler: Ian's famous "Shoot to kill?" line in ''The Medusa Plot'' is suddenly not so funny anymore after Evan is ''actually'' shot and killed at the end of the SequelSeries. After he and Ian seemed to have been starting to develop a friendship, no less.]]

** [[KilledOffForReal The killing of minor and major characters]] ([[spoiler:William [=McIntyre=], Erasmus, Luna Amato, Alistair Oh, Natalie Kabra, Evan Tollier, Isabel Kabra and Damien Vesper III]]) was criticized for being [[WhatDoYouMeanItsForKids too dark]], [[SacrificialLion cheap ways to raise the stakes]] and [[TheyWastedAPerfectlyGoodCharacter complete wastes of interesting characters]]. In this series, while there is only one minor character who dies ([[spoiler:Pony]]), no other significant characters bite the bullet by the end, which was praised, because the stakes were still high without needing to resort to pointless deaths.

** The Vespers crossed it at the start of the story arc by kidnapping the Cahills' loved ones (including both of Amy and Dan's legal guardians) to {{Blackmail}} them into committing a series of heists across the globe.
** [[CorruptPolitician J. Rutherford Pierce]] crossed it with his plan to detonate a series of hidden nuclear bombs across the planet, kickstarting World War 3 and killing countless innocents. With the world in chaos, Pierce would dominate the globe with his army of serum-addled SuperSoldiers, all while painting himself as a savior. Even after his plan is ruined, Pierce refused to disable the nukes, instead opting to let them detonate out of spite.
** [[EvilOldFolks The Outcast]] not only attempted to recreate four historical disasters to challenge the Cahills; [[spoiler: he [[BeenThereShapedHistory deliberately caused the original]] UsefulNotes/{{Chernobyl}} disaster as a smokescreen for his own illegal activities]].

** [[CorruptPolitician J. Rutherford Pierce]] crossed it with his plan to detonate a series of hidden nuclear bombs across the planet, kickstarting World War 3 and killing countless innocents. With the world in chaos, Pierce would dominate the globe with his army of serum-addled SuperSoldiers, all while painting himself as a savior. Even after his plan is foiled, Pierce refused to disable the nukes, instead opting to let them detonate out of spite.

** Isabel Kabra crossed it when she blackmailed the other Cahills into giving her their clues by taking their loved ones hostage. She even shot her own daughter in the foot and [[spoiler:as Vesper Two, was partially responsible for her abduction]].

** J. Rutherford Pierce crossed it with his plan to detonate a series of hidden nuclear bombs across the planet, kickstarting World War 3 and killing countless innocents. With the world in chaos, Pierce would dominate the globe with his army of serum-addled SuperSoldiers, all while painting himself as a savior. Even after his plan is foiled, Pierce refused to disable the countdowns for the nukes, instead opting to let them detonate out of spite.
** The Outcast not only attempted to recreate four historical disasters to challenge the Cahills; [[spoiler: he [[BeenThereShapedHistory deliberately caused the original]] UsefulNotes/{{Chernobyl}} disaster as a smokescreen for his own illegal activities]].

* J. Rutherford Pierce crossed it with his plan to detonate a series of hidden nuclear bombs across the planet, kickstarting World War 3 and killing countless innocents. With the world in chaos, Pierce would dominate the world with his serum-dosed army of SuperSoldiers, all while painting himself as a savior.

** The Vespers crossed it by kidnapping the Cahills' loved ones (including both of Amy and Dan's legal guardians) to {{Blackmail}} them into committing a series of heists across the globe.

** The Outcast not only attempted to recreate four historical disasters to challenge the Cahills; [[spoiler: he [[spoiler: deliberately [[BeenThereShapedHistory caused the original]] UsefulNotes/{{Chernobyl}} disaster as a smokescreen for his own illegal activities]]. That is, if [[spoiler: [[AnimalTesting experimenting on live animals]] with [[ILoveRadiation radiation]] in an effort to recreate the effects of the Serum]] didn't push him over the edge.

** Isabel Kabra crossed it when she blackmailed the other Cahills into giving her their clues by taking their loved ones hostage. She even shot her own daughter in the foot.
* The Vespers crossed it by kidnapping the Cahills' loved ones (including ''both'' of Amy and Dan's legal guardians) to {{Blackmail}} them into committing a series of heists across the globe.
** The Outcast not only attempted to recreate the UsefulNotes/{{Chernobyl}} Disaster to test the Cahills; [[spoiler: he [[spoiler: deliberately [[BeenThereShapedHistory caused the original]] as a smokescreen for his own illegal activities.]. That is, if [[spoiler: [[AnimalTesting experimenting on live animals]] with radiation in an effort to recreate the effects of the Serum]] didn't push him over the edge.

** The Outcast crossed it long before the events of the series by [[spoiler:[[BeenThereShapedHistory orchestrating]] the UsefulNotes/{{Chernobyl}} Disaster to cover his tracks of illegal activity in the area]]. That is, if [[spoiler: [[AnimalTesting experimenting on live animals]] with radiation in an effort to recreate the effects of the Serum]] didn't push him over the edge.

** Isabel Kabra crossed it when she blackmailed the other Cahills into giving her their clues by threatening to kill their loved ones. She even shot her own daughter in the foot.

** Natalie Kabra. The first series had intriguing ideas for her: being someone who learnt to trust her older brother and realising that her branch, mother and father were not as kind or well-intentioned as they were. The second series ends up making her nothing more than a hostage for the Vespers, and only uses her for comedic purposes, such as her horrified reactions when she had to remove a bullet from Nellie. Even worse, [[spoiler:her death at the end of that series is [[UndignifiedDeath undignified]]: grabbing the IdiotBall by whacking the activated Machina Fini Mundi machine with a pipe, causing her to be electrocuted]].

** The Outcast crossed it long before the events of the series by [[spoiler:[[BeenThereShapedHistory orchestrating the]] UsefulNotes/{{Chernobyl}} Disaster to cover his tracks of illegal activity in the area]]. That is, if [[spoiler: [[AnimalTesting experimenting on live animals]] with radiation in an effort to recreate the effects of the Serum]] didn't push him over the edge.
** Mr. Burnam; the Outcast's incompetent butler, crossed it when he dropped Saladin off at a pet shelter to be euthanized, then sadistically gloated about it to Amy.

** The Outcast crossed it long before the events of the series by [[spoiler:orchestrating the Chernobyl Disaster to cover his tracks of illegal activity in the area]]. That is, if [[spoiler: [[AnimalTesting experimenting on live animals]] with radiation in an effort to recreate the effects of the Serum]] didn't push him over the edge.

** Mr. Burnam; the Outcast's [[BumblingHenchman incompetent butler], crossed it when he dropped Saladin off at a pet shelter to be euthanized, then sadistically gloated about it to Amy.

* ToyShip: [[FanPreferredCouple Dan/Natalie]], Dan/Reagan, Dan/Madison, [[HoYay Dan/Atticus]]...

** At least towards the end of the series, [[spoiler: Ian and Natalie.]] In the beginning, they were quite cruel. But after we meet [[spoiler: their mother...]]

** Dan and Amy.

* MindGameShip: [[OppositesAttract Amian]], in canon.

** In canon itself, [[BadassBookworm Amy Cahill]]--Ian Kabra, [[GirlOfTheWeek Kurt]], [[NiceGuy Evan Tolliver]], [[JerkWithAHeartOfGold Jake Rosenbloom]] (to say nothing of {{Fanon}}, which has shipped her with [[TheBigGuy Hamilton Holt]], [[WellDoneSonGuy Jonah Wizard]], and [[TheSmartGuy Ned and Ted Starling]], among others). Then, in fanon, we have [[TeenGenius Sinead Starling]]. The most popular Sinead ships are [[BrainsAndBrawn Sinead/Hamilton]] and [[SnarkToSnarkCombat Sinead/Jonah]], but she has also been shipped with Casper Wyoming and occasionally [[PairTheSmartOnes Ian]], and even fanfiction shipping her with [[AnnoyingYoungerSibling Dan Cahill]], [[BrotherSisterIncest Ned]], or [[LesYay Amy]] have made appearances.

* HoYay:
** Fiske and [=McIntyre=].
** Dan and Atticus, and ''how''.
** Jonah and Hamilton.

* FanonDiscontinuity: ''Many'' fans would like to pretend that ''Day of Doom'' never happened, [[OutOfCharacterMoment for]] [[RomanticPlotTumor multiple]] [[KilledOffForReal reasons]].