Dyson and Tamsin survived the car crash, but all of a sudden no one (save Kenzi) could remember Bo. To bring her back, Kenzi destroyed an artifact that belonged to The Collector (who was really George Takei with a tail) which lifted the fog and Bo's remembered once more! But where WAS Bo?
Well, Bo woke up on some weird cosmic death train. She had no idea where the hell she was or why she was there, and frankly neither did we. Meanwhile, back in the real world, Dyson enlisted the help enlisted the help of Clio, a nymph, to help find Bo.
While on the cosmic death train, Bo ingested the Chi of a Jumper (more on that here) a malevolent Body Jumping Fae. Because Bo reunited the tortured Jumper with her true love, the Jumper did Bo a solid and warned her that Clio had sold Bo out to the Morrigan (the head of the Dark Fae, aka a big baddie). By that time, Clio (played by Mia Kirshner and seen here) and Dyson had found Bo, but a hip Bo drained Clio good. Then she and Dyson ditch the cosmic plane and head home.
Catching up with our other players: Following the car crash, Tamsin briefly de-aged but grew up again (and sprouted wings). It's a side effect of a Valkyrie’s protracted maturation cycle. Kenzi, in love with Dyson's BFF Hale, had an unfortunate run in with the Una Mens, the keepers of the Blood Laws between Light and Dark Fae. Think of them as really weird, emotionless cops.
So, all Fae must align with either the Dark Fae or the Light Fae. Bo has always been vehement about never taking a side. The Una Mens told Bo that they (unbeknownest to her) had tested her blood and decided she was Dark Fae. Bo was sort of like "Screw you, I'm a free agent," which pissed the Una Mens off.
The five Una Mens later summoedn Trick to help them find the Blood King, which WE know is Trick but they didn't. They wanted the Blood King so they could rob him of his soul and become one unstoppable super Fae cop power team.
Bo and Dyson were caught in the act - yes, that act - by the Una Mens and Light Fae Dyson was promptly punished for playing in the Dark (pun intended). Bo and Kenzi teamed up to rescue Dyson (it involved a red string on Dyson's ankle. Don't ask). The Una Mens just got more and more pissed at Bo and crew.
Bo wanted answers on her abduction, so she ventured back to the weird cosmic death train to search for The Wanderer, a powerful and evil Fae who had Bo kidnapped last season. While there, Bo met Rainer, a Fae who was banished to the weird death train back in the day when he fought against the whole idea of a Light Fae/Dark Fae classification system.
Bo became determined to save Rainer's soul, which was sorta damned ever since being banished to the death train. The two infiltrated the Una Mens' lair (since they basically banished Rainer in the first place, they are the key to getting it back). A scuffle ensued and the Una Mens were eliminated, but not before they warned Bo that Rainer was the spawn of evil.
Trick echoed the sentiment of the Una Mens when it came to Rainer. And with the Una Mens dead, all of their power was returned to the Origin Seed. Trick, as the Blood King (a.k.a. a very important Fae), was supposed to get it. But before Trick got his hands on it, the Origin Seed was stolen. Whodunnit?!
Bo and Kenzi prepared to take revenge for Hale, but Bo got sidetracked when she found out that Reiner had a curse on his head that must be lifted in seven days. So, basically Kenzi was on her own in the avenging Hale department.
Kenzi enlisted Vex, the former Dark Fae rule enforcer, to help her kill Massimo once and for all but Vex revealed that he was Massimo's surrogate father. Seriously. It's like The Young and the Restless but with supernatural druids.
Massimo, who was actually the Morrigan's son (!!), tried to prove his worth to his mom by kidnapping Lauren and then presenting mommy Morrigan with the Origin Seed (he stole it!). He swallowed the seed, which made him super strong and even crazier, which was actually possible.
Rosette, this Knight that Rainer worked with back in the day, was witness to the Bo/Rainer wedding and started laughing like a crazy woodpecker once the starcrossed couple said the Fae equivalent of "I Do." Turns out, Rosette was working for "The Lord of Darkness" (subtle, right?) and Bo/Rainer's marriage allowed him to be freed. Then Rosette jumped into a fire and burned herself alive because this is Lost Girl and things like that happen.
Bo and Rainer formed an uneasy alliance with Trick (who didn't trust Rainer) to try to prevent the Lord of Darkness from escaping Hel (yes, only one L). Also - the Lord of the Darkness was also probably the Wanderer (the bad dude that wanted Bo kidnapped last season). Plus, he was probably Bo's father!
Bo was hearbrokendover the loss of Rainer. Dyson, who apparently doesn't have a jealous bone in his body, vowed to stand by her. The comrades entered the temple of the priestesses of Epona where the portal to Hel had opened and which they tried to close up. Given that Hel is all dark and evil, and Bo had Dark Fae tendencies, she momentarily goes all dark side but Dyson straightens her out. During all this, Massimo unintentionally killed his mom, The Morrigan.
Meanwhile, Kenzi discovered an ancient passage foretelling that Bo must sacrifice her heart in order to seal the gates of Hel and prevent the rising of her father. Knowing that there's no bond like a BFF bond (i.e. Kenzi is "her heart"), Kenzi waleds serenely into the portal's mouth and succumbed to a warrior's death.
With Hel sealed off thanks to Kenzi, a grief-stricken Bo vowed to go to Hel and get her best friend back. Also, isn't it awesome how Kenzi's gravestone just highlights her nickname more than her actual birthname?