Giles: Because it has to be.
A major disaster strikes: Cthulhu stomps through a gate, a critical bulwark to civilization is destroyed, or some person central to reality is killed. The lead characters have a plan for how to unwind time to fix it: building a time machine, performing a magic ritual, or something similar. In the process of pursuing the necessary MacGuffins and dodging mooks, the ranks of our heroes dwindle, but they lay down their lives willingly, knowing that as long as the plan succeeds everything will be put back... including them. That foreknowledge of the coming reset is key to this trope.
This is a mashup of Reset Button and Suicide Mission, obviously enough. In general this is also a subset of Dwindling Party, though it's possible that the characters may sacrifice themselves as a group. In contrast with Climactic Battle Resurrection, where the return of deceased characters to life is the prelude to a massive battle, in many cases of this trope nobody has any idea anything happened after the original disaster is undone. However, don't be surprised if some characters have Ripple Effect-Proof Memory to remember it all in the end.
Related to Expendable Alternate Universe; in this case, it's an expendable alternate timeline, but the effect is the same: it doesn't matter who has to die because, once the mission is complete, the dead characters won't be the "real" ones.
Since this is a Death Trope, expect spoilers.
- Star Trek: Early Voyages: In the Alternate Timeline story arc "Futures", the joint mission of the Enterprise-A and the Bounty to return Mia Colt to 2254 via the Well of Tomorrows on Algol II turns into one as the planet is located in a disputed system deep in Klingon-controlled space. In space, the Enterprise engages in battle with a fleet of Klingon birds-of-prey. Sulu dies when his console explodes. On the surface of Algol II, Scotty is enraged by the death of his Bounty crewmate Yssir and is killed with a Klingon bat'leth when he attempts to retaliate. In the meantime, the Enterprise has been severely damaged by the Klingon ships and many members of the crew, including Saavik, are dead. Just as Mia Colt jumps into the Well of Tomorrows, Captain Pike activates the self-destruct in order to take as much of the Klingon fleet with him as possible.
- X-Men: Days of Future Past: The X-Men in a Bad Future send Wolverine into the past with Mental Time Travel to Set Right What Once Went Wrong, and perform a heroic last stand to protect his body and buy him enough San Dimas Time to succeed. They all die, but history is changed just in time to save Wolverine's body from destruction, and when he wakes up back in it, he's in a new timeline where they all survived (as did a bunch of other people who'd died in the old timeline).
- Edge of Tomorrow double subverts it. Cage has the ability to rewind time by a day every time he dies through most of the movie, effectively making his whole mission this. But near the end, he loses the ability to reset, and leads a team to finish the Omega Mimic once and for all, sure enough, with them dying to the last man. But by killing the Omega, he regains the ability and shifts back two days, with the Omega retroactively dying and preventing the whole plot.
- Buffy the Vampire Slayer runs with this in "The Wish." Cordelia, in a fit of jealousy, wishes that Buffy never came to Sunnydale, a wish granted by Anyanka, a Vengeance Demon masquerading as a high school student. In the resulting Alternate Universe, Buffy never came to Sunnydale, never defeated The Master, and as a result the town is overrun with vampires, with Buffy's best friends, Willow and Xander, now being the Master's sadistic vampire lieutenants. Most of the cast die in the final fight, with the page quote above happening just before Giles destroys Anyanka's amulet, resetting the universe (and incidentally leaving Anyanka stuck as a human teenager struggling to make it through high school. Don't overthink it.)
- Lost: Deconstructed in Season 5. The characters in 1977 undertake the extremely reckless mission to detonate a hydrogen bomb on top of the electromagnetic energy pocket that will cause their plane to crash on the Island thirty years later, racking up a huge number of casualties on the way, on the assumption that it will all be undone when their mission succeeds. In the end not only does the mission fail, it's implied to have caused the very disaster they were seeking to prevent, and results in Pierre Chang losing his arm, along with the deaths of a large number of DHARMA Initiative workers, Daniel, Juliet, and Sayid, who becomes a servant of the Man in Black for most of the final season after he is seemingly resurrected by him.
- Seven Days: This is a regular occurrence, where the team will work to send Frank back in time. In the second episode 98% of the world's population, including all the main characters except Frank, die due to a virus outbreak/attack. They work to get Frank and the sphere back in time, knowing that if he succeeds they'll all be fine.
- Star Trek: Deep Space Nine: In the episode "The Visitor", Benjamin Sisko is trapped in subspace by a Negative Space Wedgie and periodically comes back for a few minutes at a time into the life of his son Jake, at ever increasing intervals. Jake abandons his writing career and spends his whole life trying to find a way to save his father. Eventually he determines that the tie between them can only be severed at the point of Jake's death, but that the outcome depends on Sisko's location at the time of Jake's death; if he's in subspace, he'll be lost forever, but if Jake's death occurs in one of the rare moments where Sisko is pulled into normal space, it will reset time to the time where the link was first formed — the accident. He poisons himself so that he'll die at the exact time of Benjamin's next visit, sever the tie, undo about sixty years, and put Benjamin back a few seconds before the anomaly, giving him time to jump out of the way.
- Star Trek: The Next Generation: In the episode "Yesterday's Enterprise" the Enterprise-D sacrifices itself to cover the return of the Enterprise-C back to its original time in order to prevent the alternate timeline it emerged into from occurring.
- Star Trek: Voyager: The "Year of Hell" two-parter has Janeway go up against a timeship that has the power to Ret-Gone people and things. Voyager's shields turn out to block the time wave, allowing them to recognize the changes. At the end, Janeway uses Voyager to ram the timeship, causing it to Ret-Gone itself, erasing the eponymous year and setting all of history back in order.
Janeway: Time's. Up.
- Star Trek: Enterprise: In the episode "Twilight", an alternate future is shown where Humanity is all but wiped out by the Xindi, due to Archer losing his ability to form long-term memory, and thus not being able to lead the Enterprise crew to stop the Xindi from attacking Earth. When it is discovered that the parasites causing Archer's illness can be eliminated from the past by eliminating them from the present, the surviving members of the crew (after most of them are killed when the bridge is destroyed) attempt to complete Archer's treatment in order to change the past. As the ship is invaded, they gradually get killed off - until Archer realises that if he is destroyed, so will the parasites, thus using his dying moment to trigger the ship to self-destruct, destroying him and thus the parasites, and resetting the timeline.
- Stargate SG-1:
- In the episode "2010", the heroes attempt to send a note backward through time to prevent the covert sterilization of humanity by what were believed to be friendly aliens. All four of them are killed, but they manage to send back a cryptic clue which eventually prevents involvement with the aliens, and thus the timeline that lead to their deaths.
- In the final episode "Unending" the titular SG-1 are trapped between two Ori battleships. Samantha Carter freezes time outside of the ship. They grow old on board and eventually Carter works out a way to escape — the twist being, they use the projectiles as an energy source to go back in time (or something like that), but one of them has to stay old. They choose Teal'c, the Token Non-Human, since he ages very slowly, and the ship is blown up by the laser. Luckily, it works, so Teal'c is sent back in time to put in a special program Carter wrote, allowing them to escape.
- 12 Monkeys: This is essentially the whole plot, as Cole and the rest of Team Splinter acknowledge from the start that succeeding in averting the plague will essentially wipe out the current versions of themselves. Ultimately, for Cole it becomes a literal suicide mission, as it turns out that the only way to break the series of Stable Time Loops is to erase himself from having ever existed in the first place.
- Warehouse 13: At the end of Season 3 the Warehouse is destroyed, important supporting characters were all dead, and the world was coming to an end. At the start of Season 4 the heroes can rewind time by 24 hours if they get the right magical goodies, but a secret society takes exception to it. The heroes drop like flies, but no problem, it'll all be fixed in the rewind.
- Legends of Tomorrow: Towards the end of Season 2, the Legion of Doom get the Spear of Destiny and use it to create Doom World. The Spear is then destroyed by Thawne, preventing the Legends from using it to fix reality. Out of options, they double back on their own timeline and try to keep the Spear from being taken by the Legion in the first place. They acknowledge that doing so will cause them to cease to exist, and deem it an acceptable price. All of the Doom World Legends except Sara die in the battle (several ensuring their counterparts survive), and once the Spear is destroyed she peacefully fades from existence.
- Dragon Age: Inquisition: In the main quest "In Hushed Whispers", the Inquisitor and Dorian are propelled into a Bad Future where the Big Bad has won. Their former companions and Leliana then sacrifice themselves to return the Inquisitor and Dorian back to the present, preventing this future from ever occurring.
- Ghost Trick: After being stuck on a sinking submarine, Sissel and Missile say their farewells to Lynne and Kamila before hitching a ride on a torpedo towards a jettisoned submarine room. Said room contains their ticket to the past, so that they can rewind time once more and undo the death that started it all.
- A Hat in Time: In the finale, the Big Bad has rewritten time to put themselves in charge. In a tantrum, the Big Bad kills off several minor characters, which causes the characters to burst into healing Pons. One of the Mafia realizes that they can help Hat Kid win by sacrificing themselves to create healing Pons for Hat Kid, and if Hat Kid wins, time is reset to what it should be, bringing them back too. This results in a large number of supporting characters punching each other out to keep Hat Kid topped off on health during the final phase of the battle.
- Schlock Mercenary: One story arc revolved around trying to undo a false-vacuum collapse that will obliterate space itself. The same disaster made time travel possible when ordinarily it isn't, so Kevyn is chosen to go back in time with the information that will prevent the disaster (and incidentally prevent the death of Captain Tagon). The physics of the time gate requires those left behind to keep it open as long as possible in order to send Kevyn as far back as possible; they all die doing so to give Kevyn a chance.
- SCP Foundation: SCP-5000 tells the story of a Foundation operative who went on a journey across the world on a mission to reset the universe after the Foundation discovers something that leads them to believe that ending humanity is the best course of action and wreaks havoc. He succeeds, but falls to his death in the process and the story begins with his remains appearing in a flash of light in the new universe with his Apocalyptic Log.
- Justice League: one episode sees Batman, Wonder Woman, and John Steward travel to a Bad Future, where they team up with the future League to prevent it from ever occurring in the first place. Many of the future League members die in the battle, but the proper timeline, as seen in Batman Beyond, is ultimately restored.
- Averted in Code Lyoko, where it's explicitly said that even the "Return to the Past" ability of the supercomputer can't bring back the dead. Doesn't stop it from being activated when people are on the verge of being killed, though.
- Avengers, Assemble!: In one episode, the Squadron Supreme uses the Reality Gem to create a reality where they're heroes and the Avengers are bad guys. Being aware of this by the time it seems he'll have to sacrifice himself to save the day, Iron Man does so in the express belief he'll be restored along with the original reality. He's proven right.
- Zecora pulls this in My Little Pony: Friendship Is Magic in a Bad Alternate Future that's ruled over by Queen Chrysalis. She knows that Twilight Sparkle can set the timeline right again and prevent it, and goes down fighting Chrysalis in a surprisingly dark scene. You know, for kids!
- The start of the series finale of Xiaolin Showdown is kickstarted cause Omi tried to go into the future to see his future self forgetting that if he wasn't in the past, he wouldn't be in the future. And since he took Dojo with him, the other monks weren't able to find the Shen Gong Wu allowing Jack Splicer to get them all and take over the world (Yes Jack, the bumbling idiot villain rather then more powerful Wuya, Chase Young or Hannibal Bean. Who knew?). Omi reunites his fellow monks, who obviously are old by this point, who help him get into Jack's fortress and get the Shen Gong Wu needed to go back in time and prevent this. However of course they end up meeting their end doing so just as Omi (or rather Dojo as Omi was in shock at witnessing this) activates the Wu.