Plot armour

The plot armour carried by characters is a significant concept in Syphon Filter games. Essentially, this prevents the killing of characters who contribute in some sense to the storyline of each game. Plot armour is either a type of protection given to them granting them almost complete invincibility unless a certain weapon is used, or a game mechanism whereby permitting their death before they are supposed to perish would cause a certain objective to automatically be failed (the latter of which can sometimes lead to a mission ending).

Notable examples

Syphon Filter

The CBDC personnel who are seen during the Washington DC Attack must be kept alive, and killing them will cause mission failure.
Mara Aramov is somehow always immune to headshots when Gabe Logan shoots her. This applied until Dark Mirror, when Stone finally assassinated her for good.
Anton Girdeaux cannot be killed with any weapons, due to his advanced body armour. Even explosives cannot be used, in order not to provoke the bomb into detonating. Only if Logan shoots his fuel line will he be defeated.
During the first investigation of PHARCOM, shooting Phagan will result in mission failure.
When he attempts to stop Mara from killing Phagan, Gabe must use non-lethal force against both.
It is not technically a plot armour issue, but while destroying Rhoemer's base, Lian is 'killed' by the antagonist. However, should Gabe shoot the fuel tanks, she can be heard on radio, telling him to watch his fire. This could simply be a developer oversight.
When he assaults Rhoemer's cathedral, Gabe cannot shoot the test subjects to whom he has to inject an antigen into. Doing so will cause mission failure.
The scientist who appears at the start of 'Stronghold Catacombs' must be kept alive, since he knows the way to Phagan's cell.
Phagan is shot in the head and seemingly killed, but he does not truly perish until the next game.
Thomas Markinson can never be encountered in gameplay, and is only seen in cutscenes. It is thus impossible for Gabe to kill him.
Rhoemer is scripted to have full body armour and even a helmet, despite visually having no such protection.

Syphon Filter 2

When searching for the data discs, the GI's are scripted to be killed by enemy Spooks. If Gabe shoots them, he will trigger a mission failure. This also applies to Chance.
Mr Cochrane, as with Markinson, is invincible (at least in gameplay) due to the fact that only cutscenes feature him.
When escaping from MacKenzie Air Force Base, Lian cannot kill airmen and must use non-lethal force on them.
Phagan is somehow unaffected by headshots, as with Mara, in that Aramov's shot in the first game merely left him brain dead.
On the I-70 Mountain Bridge, killing the bombing commander with an unsuppressed weapon causes mission failure.
The doctor who approaches Lian in McKenzie Air Force Base is only knocked out but not killed (at least, the player doesn't see him perish although he could be killed off-screen).
It is considered a breach of parameters to kill any security soldiers in the air force base.
When she interrogates Holman, Lian can only wound him.
After reaching the C-130 wreckage, Gabe must kill Archer with a headshot. He is immune to any other attack for gameplay purposes.
During the second investigation of PHARCOM, Gabe cannot kill any GI's.
When confronting Morgan, the player cannot kill him until Teresa diffuses all the bombs. When finally fighting him, Logan must use a headshot or grenade against him.
When pursuing Uri Gregorav, Lian cannot kill the Russian militia or the former.
During the rescue of Gregorav, Lian must use nonlethal force against prisoners.
When he acquires Lian's vaccine, Gabe cannot kill Gershon, Chance or Ramirez.
When escaping from the bio-labs, Unit One personnel are completely immune to bullets.
The NYPD SWAT personnel cannot be killed when Logan is escaping from New York.
Teresa must be kept alive during the escape from New York.
Chance is, like Rhoemer, immune to headshots.

Syphon Filter 3

Civilians cannot be killed during the assassination of Shi Hao.
Lian and villagers must all be kept alive during the Costa Rican Incident.
When confronting Rhoemer, he is immune to almost every weapon: as with Chance, only the USAS-12 will be effective.
During the Pugari Outbreak, Mujari has to protect slave workers and team members.
Although the player is not penalised for killing civilians during the first destruction of the Lorelei, ignoring them will convey multiplayer bonuses.
When investigating Rhoemer's activities, Lian must protect the scientists she encounters.
Maggie Powers and MI6 operatives must all be kept alive in 'Waterfront'.
Nigel Cummings must be protected during the first part of 'Docks Final Assault'.
Both Ellis and the truck must survive in 'Convoy'.
In 'The Beast', the entire convoy is required to survive in order to proceed to the next mission.
During 'Australian Outback', Elsa Weissenger must be kept alive.
The aborigines must be protected during 'St. George, Australia'. As such, Lian must maintain stealth until Commander Silvers is assassinated.
Because Teresa was shot in the previous game, most characters assumed her truly dead until she reappeared, explaining that she faked her death.
In the Paradise Ridge Incident, unarmed militia members and federal agents cannot be killed.
During 'Militia Compound', Silvers is scripted to only be bugged, since killing him would cause mission failure due to a time paradox; he can only be killed by Lian. ATF and militia members must also be protected.
The Oakton family cannot be killed for 'Underground Bunker' to complete this successfully.
Teresa must administer terbutaline to Debbie Oakton, and failing to perform this will fail the mission.
Federal agents and civilians must be protected during the second apprehension of Mara Aramov.
Vince Hadden is only seen in cutscenes, and the player can never kill him, much as the case was with Markinson and Mr Cochrane. It is ultimately Mara who shoots him before the last mission.

Syphon Filter: The Omega Strain

The CHA personnel during the ALA Assault on Carthage must be protected for this objective to be complete.
Although Imani asks for covering fire when she rescues Mujari, she remains invulnerable against anything. She is only killed by the SLORC when she shadows Yong-jun Kim on a flight that crosses Myanmar.
Richard Broussard, despite being antagonistic, is never seen in gameplay, although a cutscene features him in the quarantine zone. The ALA attempts to kill him with a car bomb, but this fails, and he is only slain by Masson in a cutscene.
During the 'Krivorozhstal Mill' mission, killing the workers will fail the parameter for 'no collateral damage'. Zhidkov is invincible in this mission as well: he can only be killed with a C4 charge on the train tracks. Strangely, using even non-lethal weapons on the workers will count as killing them.
Alima Haddad is presumed killed during 'Belaya Vezha'. It is only at the game's end that she is revealed to be alive.
Askar Saydahmat is absent during 'Saydahmat's Village', and Lian must poison his water chalice. Furthermore, Samaev must only be killed when he has the bio-weapons container. Killing Yang before the deal also fails the mission.
It is strange, but if Zohar is killed during 'Taherir Palace', the mission fails automatically yet he appears alive and well in the copter when the mission ends. It is possible that he is supposed to stay alive in cutscenes regardless of what actually happens in-game.
- Furthermore, Fatha al-Hassan is never seen in gameplay and only assassinated by Zohar off-screen.
During 'International University', the security personnel must be kept alive. Furthermore, the armoured guards that appear when the player escapes from the institute share a trait with Girdeaux: wearing full body armour, they are invulnerable unless players attack their backpacks.
Despite the fact that Mujari is allowed to kill soldiers (which goes against his personal vow of not using lethal force except in self-defense) in the Pugari Outbreak, he must use non-lethal force against enemies in 'Ivankov's Home'. Gregorav also cannot be killed.
Salvage personnel must be kept alive in 'Lorelei Salvage Rig'.
Sanzo Murukawa commits suicide, despite Lian attempting to capture him alive.
Iwao Ryusaki cannot be hurt by gas grenades, due to his face mask. This also applies to the lab employee.
As with 'Ivankov's Home', the bank security in 'Niculescu Funds Tower' can only be neutralised with non-lethal force.
It is also not a plot armour issue, but Gabe swears not to use lethal force against Niculescu's mercenaries until he has evidence that the art collector is indeed connected to Syphon Filter, yet he begins the mission with a sniper rifle and it is possible to complete the mission by killing all the guards.
As with Jason Chance, despite the fact that Mara Aramov is a major antagonist, she must survive in 'Niculescu's Villa Estate'. Only Stone will kill her in the game's epilogue.
For gameplay reasons, Stone and Gabe are all invincible during 'Chechen Terrorist Base', despite the former asking for covering fire when he rescues Alima.
Like Hadden, Mihai Niculescu is never encountered in gameplay, only in cutscenes. As such he cannot be killed until criminals shoot him as a result of the IPCA recruit planting a computer virus that wiped out his bank's records.
Mikhas Ivankov cannot be encountered during gameplay as well; he is only seen in cutscenes. As such, although this sequence was never seen, John Garvin confirms that Gabe personally kills him following the skirmish between the IPCA and CDP.

Syphon Filter: Dark Mirror

As with Girdeaux (and the armoured guards in Minsk, Belarus: International University), Red Jack cannot be killed with conventional weaponry, and Logan must detonate his fuel tank to successfully defeat him.
In 'A Man Without Power' and 'Freeman's Files', killing the electrician or otherwise allowing his death causes mission failure.
Similarly, allowing Kriesler to die or killing him in 'Kriesler's Garden' also fails the mission; he is only scripted to die from a white flower he sniffed after Gabe questions him.
Furthermore, the flamethrower-armed enemy who is seen in this level can only be killed by shooting the fuel line.
Black King, like Morgan, can only be killed when the explosives are diffused.
Lian must survive in 'Freeman's Answer'.
Morgan Freeman is also invulnerable since he is never seen until the mission's end; by then, he has committed suicide with a cyanide capsule.
Addison Hargrove and civilians cannot be killed in 'Old Friends', as well as 'Memories'.
Because Gabe attempts to rescue Janzen in 'Forged Under Fire', the private must survive until 'The Ultimate Sacrifice', when Kress is scripted to kill him.
Similarly, Richard Kress is supposed to die only in 'The Trojan Horse', and killing him prior to that causes mission failure, even though, as with Chance, he is a treacherous operative.
Ritko Boravnic is killed by an unknown person, and Gabe will only be able to see his corpse.
Goran Zivmovic can never be killed since the player only sees him in cutscenes. It is not until 'The Trojan Horse' that he is killed from the tank's explosion.
Killing civilians in 'Fist Full of Rubles' causes mission failure.
Like Teresa between Syphon Filter 2 and 3, Addison Hargrove fakes her death in 'Meeting with Fate' despite seemingly having been killed, and reappears at the game's epilogue.
Allowing civilians to die in 'Meeting with Fate' fails the mission.
If the elevator falls in 'Into the Abyss' and the innocents are still inside, the mission is failed.
Lian must be kept alive in 'Red Section' so that she can infiltrate AIT in 'Drowning'.
The personnel who wear AS armour are totally invulnerable against conventional attacks, and must be hit with an electric attack, which deactivates their protective equipment, before the player can use firearms on them.
It is not no legitimate plot armour issue, but shooting Blake in 'Drowning' does not affect her whatsoever and only if she suffocates will she be killed. This could be due to storyline reasons, for she must survive to have the mission continue.
Only Lian is able to kill the first guard in 'Event Horizon'.
As with 'Red Section', Logan must cover Lian during 'Event Horizon'. Permitting her death causes mission failure.
In 'KemSynth Tower', the Red Section leader is only vulnerable to attacks aimed at his fuel line.
In 'Bangkok Part 2', allowing Lian to die causes mission failure.

Syphon Filter: Logan's Shadow

Alima Haddad is only scripted to be killed during 'Going Under'. If the helicopter is shot down prior to this point, the player fails the mission.
Dane Bishop must also stay alive during Episode 2. Permitting his death will cause mission failure.
During the fight against Surgeyev, breaching the reactor causes automatic mission failure.
The resistance leader in 'Missing Friends' has to survive, otherwise mission failure is also triggered.
Failing to press the correct button sequence in 'Found Enemies' allows Trinidad to cut Gabe's neck with her knife, which causes Gabe's death and, therefore, mission failure.
Trinidad is invincible against Gabe's attacks in 'Found Enemies', and only Spetsnaz members can hurt her. If she dies, this triggers mission failure.
Should the Spetsnaz copter kill Maggie Powers in 'Nowhere to Run', the player fails this mission. However, as with Trinidad, it is impossible for the player to harm her.
Maggie Powers is scripted to run out of air in 'Trinidad' when the timer for oxygen hits zero, and Gabe must rescue her to allow the mission to progress.
Maggie is also scripted to survive during 'Deadly Cargo', thus the player cannot allow the cockpit breach to happen.
The tank commander in 'Operation Canyon Storm' must survive until the mission ends.
Boomer is scripted to die only at the end of 'Our Hidden Past'. If enemies kill him while he sets the explosives, the player fails this mission.
It is also a requirement that Lian survive in 'Desert Flames'. Should she be killed, the mission is failed.
As with Dark Mirror, enemies with AS armour are invincible against normal attacks, and must be hit with an electric or explosive attack before they become vulnerable.
Shen is scripted to survive in 'Leading the Blind' and will kill himself at the game's end. The player must thus keep him alive.
Bitar is completely invincible against anything except the X-Z-2 explosion.
Lian is scripted to survive her first assignment, so allowing her death causes mission failure.
Allowing Gabe to die in 'Shadowed' fails the mission.