In the final season of Lost Girl, Bo, desperate to stop the Ancients, hellbent on using the Nyx to wipe out all of Human and Faekind, does the unthinkable: She beckons daddy dearest from the depths of the underworld. It takes evil to fight evil.  But will the man Bo's been waiting five seasons to meet turn out to be Dark Lord she's expecting, or is there any hope in hell he might be the loving father she deserves? As the truth surfaces, each and everyone one of Bo's friends are affected, and conflicting narratives force Bo to question everything she's ever known about who she is, where she comes from, and who she can trust. Bo's final journey comes to a heart-racing conclusion when her worst nightmare becomes a reality, bringing her face to face with her greatest foe yet.

While Kenzi, Hale, and Dyson, are all living their lives, Bo is nowhere to be found. It's later realized that they simply forgot Bo, as someone was forcing them to. Massimo has been giving Kenzi temporary powers to appear Fae. Bo finally awakens to find herself on a train, and later jumps off. A group of Fae called the "Una Mens" are introduced. When she arrives home, it is discovered that while Bo herself did not consciously choose a side, her blood has chosen Dark. Tamsin is found reborn, as a little girl, and grows up with Kenzi as her pseudo-mom. Massimo steals from Bo and Kenzi in an attempt to convince Kenzi to pay him, and Bo figures out that he is not Fae, but human. He also kidnaps Tamsin to acquire her Valkyrie hair, and after being defeated by Bo, chases after the hair into a pit of lava, where at that point he is assumed to be dead. Many of Trick's secrets and past actions are revealed, including a tie to a past life of Tamsin's, and the fact that he used his blood to "erase" someone from existence. Tamsin discovers that by not taking the soul of a man named Rainer to Valhalla, she is part of the reason "The Wanderer" was created. Bo is able to get back on the train, where she finally meets Rainer, and brings him back to the Dal. Hale and Kenzi admit their feelings for each other. Lauren, who has been working with the Dark, somehow turns the Morrigan human. Kenzi's mother is introduced, and Hale attempts to propose. Massimo returns, and protecting Kenzi, Hale is killed. Kenzi tries to get revenge, but is stopped by Vex, who mentions that he is Massimo's guardian. Evony is revealed to be Massimo's mother, and gave him to Vex years ago when he was a boy. Bo learns that not only is her father coming, but that to close the portal, she will need to give her heart. That is revealed to be Kenzi, who sacrifices herself. It ends with Bo visiting Kenzi's grave.

With Fae society in upheaval, Bo finds herself facing further changes and challenges as former ally Hale becomes the acting Ash – trying to forge a new balance between Light and Dark by appointing a Valkyrie aligned with the Dark Fae, Tamsin, as Dyson's new detective partner. Meantime, Tamsin is a secret agent working for two separate clients: The Morrigan, who wants to build a case against Bo so that she can execute her; and as a mercenary for someone who wants to entrap Bo. Matters become complicated when Kenzi is kidnapped by a crazed Kitsune who assumes her identity and deliberately sows distrust in the relationships between Bo and those closest to her, just as Bo must prepare for and go through an evolutionary Fae rite of passage that forces her to explore her past and future. Danger escalates when a human scientist convinces a despondent Lauren to join him in conducting scientific research in his private laboratory – all the while deceptively concealing his intent to harness Fae genetics for himself with the use of her expertise. The third season culminates with Bo being engulfed by black smoke and disappearing into thin air, presumably whisked away by her mysterious and powerful biological father (who may be "The Wanderer" that recurred throughout the season's story arc).