Seasons 1–2: Introduces Bobby Nash running Station 118 while hiding a devastating past: in Minnesota, his substance abuse caused an apartment building fire that killed 148 people, including his wife and two children. He views his firefighting service in Los Angeles as a path to make restitution. Athena Grant separates from her husband Michael after he comes out as gay, later entering a romance with Bobby; they marry in the Season 2 finale. Maddie Buckley flees her abusive husband Doug Kendall, who tracks her down and brutally stabs Chimney; in a life-or-death confrontation in the woods, Maddie kills Doug in self-defense. Buck suffers a crushed leg when a serial bomber blows up the 118 fire truck, triggering an agonizing rehabilitation arc.
Seasons 3–5: Buck gets caught in the Santa Monica tsunami alongside Eddie's son Christopher; against astronomical odds, both survive, forging an unbreakable bond. Athena survives a brutal assault by a serial rapist, leading to a long psychological journey to regain her confidence on duty. Maddie gives birth to Jee-Yun Buckley Han with Chimney, but suffers severe postpartum depression, temporarily leaving town to seek psychiatric treatment before reuniting with her family. Buck learns his parents concealed the existence of a deceased older brother, Daniel, whom Buck was conceived to save as a savior-sibling donor, finally explaining his lifelong abandonment anxiety.
Seasons 6–9+: Buck is struck by lightning, survives a coma, and gains clarity regarding his emotional purpose. Later, Buck explores his sexuality, acknowledging his attraction to men and entering a relationship with LAFD pilot Tommy Kinard. Bobby and Athena survive a catastrophic cruise ship capsize disaster reminiscent of The Poseidon Adventure. Major shifts occur across the leadership of the 118 following personal reckonings and leadership transitions, cementing the younger generation of firefighters as veterans carrying forward the station's founding legacy.