Known as the fearsome assassin 'Black Cat', Train Heartnet executes each mission with chilling precision, indifferent to the value of life. Serving the enigmatic organization Chronos, Train crosses paths with Saya Minatsuki, a mysterious bounty hunter who sparks an unexpected connection with him. Influenced by Saya's optimism, Train reevaluates his ruthless ways and embraces a new identity as an honorable bounty hunter. Despite this transformation, his former associates, especially Creed Diskenth, refuse to accept his change and vow to reclaim their lethal comrade by any means necessary. As Train navigates this dangerous shift from assassin to a principled stray, the echoes of gunfire threaten to shatter his newfound path.