Set in the Meiji era, Firefly Wedding follows Satoko Kirigaya, the beautiful eldest daughter of an earl who harbors a secret: a heart condition that leaves her with a limited time to live. Despite receiving many marriage proposals, she has turned them all down due to her illness, though she still hopes to find a husband to ease her father's worries. Her plans are shattered when she is suddenly kidnapped by a group of mercenaries. Among her captors is Shinpei Goto, a legendary and mysterious assassin tasked with executing her.

Facing certain death, Satoko makes a desperate gamble. Under the pretense of love at first sight, she proposes marriage to Shinpei. To her surprise, he accepts the proposal and vows to protect her, helping her escape from the mercenaries. The story follows the unlikely couple as they flee, with Satoko struggling to maintain the ruse while also trying to understand the enigmatic and dangerous man she has bound herself to. As they navigate this tense alliance, Shinpei's unsettlingly intense affection begins to challenge Satoko's resolve, and her initial guilt over the false engagement grows into something more complex.

The main characters are Satoko Kirigaya, a noblewoman who is far more resourceful and strong-willed than her frail appearance suggests, and Shinpei Goto, an obsessive and unpredictable assassin who takes his vow of protection with terrifying seriousness. The cast also includes members of Satoko's noble family and various denizens of the underworld they encounter. The series is produced by David Production and directed by Takahiro Kamei, with Lynn and Koki Uchiyama voicing Satoko and Shinpei respectively. Notable narrative arcs include the initial kidnapping and escape, the couple hiding in the lawless red-light district known as the Celestial Maiden Isle where Satoko must pose as a prostitute, and the gradual revelation of the conspiracies within Satoko's own family that led to her abduction.