Protection of the Timeline (Earth-Prime)

The Protection of the Timeline is the main mission of the Legends of Tomorrow.

Background
After the Time Masters were destroyed, the Legends of Tomorrow took over their position in serving as history's last line of defense, swearing to protect the timeline from anyone who threatens it.

Legion of Doom
As a Time Remnant, Eobard Thawne assembled the Legion of Doom and searched for the Spear of Destiny, which could allow him to rewrite the world as he want without the consequences of altering the timeline. This resulted in multiple time aberrations across the timeline including the Legends' interference in France during World War I. The Legends from an alternate reality from the future were also present in the event and created a Time Quake, resulting in catastrophic damage to the timeline.

War on Mallus
With the timeline being extremely vulnerable, the Time Demon Mallus was slowly able to break out of his imprisonment. Meanwhile, the Darhks began worsening the already existing aberrations due to the Legends' involvement in France, weakening Mallus' "cage". Later, the Legends was able to gather the Totems of Zambesi and confront Mallus, ridding his influence on the timeline and preventing further damage. Damien Darhk sacrificed himself in the process while Nora soon joined Time Bureau to protect the timeline.

Magical Fugitives
While battling Mallus, the Legends accidentally opened the door for the escape of countless magical fugitives, with them scattered across the timeline. With the help of John Constantine, they were able to track down each one of them but soon realize that they were merely misunderstood individuals, and not villainous monsters. The Legends created Heyworld to help the world to further understand them and defeated Neron, while altering the timeline so that the Anti-Metahuman Act of 2021 was never written.

Encores
Months after the Battle of Heyworld, the Legends were beloved by the world as famous "superheroes" of Earth. They then continued their mission in safeguarding the timeline and discovered Astra Logue's resurrections of multiple infamous individuals across time, coined as the Encores. The team searched for the Encores with the Prognosticator and able to restore the original timeline, eventually convincing Astra to give up in exchange for searching the Loom of Fate for her deceased mother.

Into the Cosmos
After defeating the Fates, the Legends discover their missing mission captain. They then located her in deep space with Gary Green abducting her for Bishop. While escaping Kayla's ship, Sara released dozens of aliens into the Temporal Zone. Sara was later killed and restored as a human-alien hybrid clone, with her rejoining the team as a powered individual and trying to undo her damage to the timeline. Their plan to restore the pod aliens' damage to the timeline was soon put on hold as they were bothered with Bishop's return, and also the destruction of the Waverider.