For decades -in particular, since the 1960's- Marvel Comics has been telling noteworthy fictional stories. The following is a list of the major plotlines, detailing such things as first appearances of characters, character deaths (though those tend to be less permanent in superhero comics) and other important developments. Note that all of these events take place within a sliding timescale, which means that they are assumed to have taken place in a period of up to fifteen years before the current date, regardless of when they were actually published.
After a trip to space, Reed Richards, Sue Storm, Johnny Storm, and Ben Grimm develop fantastic powers.
Re-emergence of Namor
05/1962
the Sub-Mariner gets his memories back after a meeting with the human Torch.
Emergence of the Incredible Hulk
05/1962
After being bombarded by gamma rays in a nuclear explosion, Dr. Bruce Banner becomes a rampaging creature.
Emergence of Spider-Man
08/1962
Peter Parker is bitten by a radioactive spider and embraces a super-hero career.
Emergence of Thor
08/1962
The thunder god Thor awakens inside the body of mortal Donald Blake.
Emergence of Iron Man
03/1963
Tony Stark creates his first suit of Iron Man armor.
Emergence of Doctor Strange
07/1963
After years of mystical training, Stephen Strange becomes Earth's greatest defender against supernatural threats.
Emergence of the X-Men and Magneto
09/1963
Cyclops, Marvel Girl, Iceman, Beast, and Angel are brought together for the first time by Professor X. They go public while fighting mutant criminal Magneto for the first time.
Emergence of the Avengers
09/1963
Iron Man, Thor, Ant Man, the Wasp and Hulk assemble for the first time.
Emergence of Nick Fury and S.H.I.E.L.D.
09/1963
Colonel Nick Fury and his espionage agency get involved in super-human affairs for the fist time.
Re-emergence of Captain America
03/1964
Steve Rogers is awakened from suspended animation by the Sub-Mariner and found by the Avengers. He becomes a member of the team.
Emergence of Daredevil
04/1964
As a young boy, Matthew Murdock was blinded in a tragic accident, which also honed his senses to super-human levels. Now an adult, he embraces a super-hero career.
Emergence of Galactus and the Silver Surfer
03-05/1966
Galactus and his herald, the Silver Surfer, plan to destroy Earth. The Silver Surfer rebels against his master and, as punishment, remains a prisoner of earth's atmosphere.
1970s
Event
Year
Developments
Kree-Skrull War
10/1971 - 02/1972
The Avengers are involved in a millennia-old interstellar war.
Emergence of the Defenders
12/1971
After several meetings, Doctor Strange, the Silver Surfer, Namor and the Hulk forms a new super-hero team.
Death of Gwen Stacy and apparent death of the original Green Goblin
06-07/1973
During a fight with the Green Goblin, Gwen Stacy is killed. Later this night, the Green Goblin apparently kills himself.
Professor X assembles a new team to rescue the original X-Men. First appearance of several major characters.
Phoenix Saga
1976 - 1977
The Phoenix is unleashed for the first time in the Marvel Universe, in the form of Jean Grey.
Secret Empire
197x
Captain America follows a conspiracy all the way to the White House.
The Korvac Saga
1978
The Avengers and the Guardians of the Galaxy track down a mysterious threat from the future.
1980s
Event
Year
Developments
Dark Phoenix Saga
1980
Jean Grey becomes the Dark Phoenix after being controlled by the Hellfire Club
Days of Future Past
1981
Kitty Pryde from a dystopic future travels back in time to prevent the event that created her timeline.
Contest of Champions
1982
The Grandmaster and Death manipulate the heroes of Earth in a game to decide the fate of The Collector.
Death of Captain Marvel
1982
The original Captain Marvel (Mar-Vell) falls victim to what the Kree call "the Blackend", or what is called Cancer on Earth.
Secret Wars
1984 - 1985
The Beyonder kidnaps Earth's heroes and villains to battle on a planet of his creation. Spider-Man finds his symbiote costume.
Secret Wars II
1985 - 1986
A contest of champions with the Beyonder
Mutant Massacre
1986
The Marauders kill most of the Morlocks
The Fall of the Mutants
1987
The X-Men are slain in Dallas during a cataclysmic battle against the Adversary.
Kraven's Last Hunt
1987
The final battle between Kraven the Hunter and Spider-Man.
Armor Wars
1988
Iron Man embarks on a mission to destroy armored suits that were created from his own stolen technology.
The Evolutionary War
1988
The High Evolutionary unleashes attacks all over the world to guide the way of human evolution.
Inferno
1988 - 1989
Demons from Limbo invade earth.
Atlantis Attacks
1989
Under the influence of Set the Serpent God, the Atlanteans launch a massive assault on the surface world.
Acts of Vengeance
1989 - 1990
Major villains unite to fight each others' enemies.
1990s
Event
Year
Developments
The X-Tinction Agenda
1990
The New Mutants and the X-men are kidnapped and taken to Genosha, which culminates in their escape; the Genoshan government is overthrown.
Armor Wars II
1990 - 1991
Iron Man continues his mission against those he believes stole his technology, but creates tensions between himself and those who trust him most in the process.
Infinity Gauntlet
1991
Thanos uses the power of the Infinity Gems to become Supreme Being.
Operation: Galactic Storm
1992
The Avengers become involved in a war between the Kree and the Shi'ar.
Magus, Adam Warlock's "Evil Side", creates evil doppelgangers of Earth's heroes to gain ultimate power.
X-Cutioner's Song
1992
The Legacy Virus is created, and Stryfe goes up against combined X-teams.
Infinity Crusade
1993
Goddess, Adam Warlock's "Good Side", recruits an army of heroes to fight her holy war.
Maximum Carnage
1993
The Carnage Family massacres downtown New York only to have Spider-man and Venom in their way.
Fatal Attractions
1993
Magneto returns more powerful and determined than ever, ripping the adamantium from Wolverine's bones and enraging Professor X.
Siege of Darkness
1993
The Midnight Sons battles Lilith in her quest to take over Earth.
Phalanx Covenant
1994
The Phalanx race attacks earth.
Clone Saga
1994 - 1996
The clone of Spider-Man, who was long thought dead, returns. His creator, the Jackal also returns to wreak havoc in Spider-Man's personal life.
Age of Apocalypse
1995
Legion travels back in time to kill Magneto, but accidentally kills his own father, Charles Xavier, creating a dystopian alternative timeline in which Apocalypse rules the world.
Avengers: The Crossing
1995
One of the Avengers is brainwashed and turns into a murderer.
Onslaught
1996-1997
The Marvel heroes died while defeating the nigh-omnipotent being, Onslaught, and later revealed that they have been transported to an alternate universe that was created by Franklin Richards
Operation: Zero Tolerance
1997
In the aftermath of Onslaught the U.S. government begins to round up mutants.
Heroes Reborn
1997
The heroes from Heroes Reborn return to the regular Marvel Universe.
Contest of Champions II
1998
Earth's heroes are abducted and brainwashed with nanoprobes to participate in a fighting competition.
Avengers Forever
1998 - 2000
Avengers from different time periods are assembled to participate in the Destiny War, a conflict between Kang the Conqueror and his future self, Immortus.
2000s
Event
Year
Developments
Apocalypse: The Twelve
2000
Apocalypse emerges once more, along with a mysterious prophesy of 12 powerful mutants capable of destroying him.
Maximum Security
2000
Fed up with Earth's interference in intergalactic affairs, the starfaring alien community quarantines the Sol System and turns Earth into a prison for the universe's worst criminals.
The Kang Dynasty
2001-2002
Kang and his son Marcus declare war on present-day Earth, killing everyone in Washington DC and briefly taking over the planet.
Infinity Abyss
2002
Mad partial-clones of Thanos try to destroy the universe.
Avengers Disassembled
2004
The Scarlet Witch goes insane and kills or incapacitates the current Avenger-roster.
Secret War
2004 - 2005
Nick Fury starts a secret war against Latveria who he believes has been selling high-tech weaponry to super villains.
Spider-Man: The Other
2005
Spider-Man goes through a mysterious, life changing event after facing the super villain Morlun.
House of M
2005
The Scarlet Witch alters reality, creating a world in which mutants are in charge.
Decimation
2005 - 2006
In the aftermath of House of M the mutant population has greatly decreased.
Planet Hulk
2006
Hulk is exiled to a far-off planet of Sakaar by the mysterious Illuminati.
Annihilation
2006 - 2007
Silver Surfer, Super-Skrull, Nova, and Ronan face the Annihilation Wave.
Civil War
2006 - 2007
Heroes are divided when the U.S. government passes the Super-human Registration Act.
Civil War: The Initiative
2007
Government sponsored heroes led by Iron Man take control in the aftermath of the Civil War.
World War Hulk
2007
The Hulk returns to Earth to exact revenge on those who exiled him. As an old enemy forms the Gamma Corps to take him down, a handful of heroes who helped Hulk in the past consider joining his side.
Annihilation: Conquest
2007
Nova, Starlord, Wraith, & a new Quasar lead a group of cosmic heroes against a new threat to the universe.
Endangered Species
2007
The 198 remaining mutants must avoid the threat of genocide as Beast searches for a solution to M-Day.
Messiah Complex
2007
Following the events of Endangered Species, a savior of the Mutant Race is found, and both the X-Men and Mr Sinister are after it.
Secret Invasion
2008
An event that features an invasion by the Skrull race which has gone unnoticed, and the secret purpose behind it.