Lady Sif returns to the show this week, but she is not herself. She’s not sure who she is, but she has to find someone named Kava…who could have a connection with Skye, and her very dangerous powers.
The title doesn’t only apply to Sif and Skye, but the web of secrets that Morse (Adrianne Palicki) and Mack (Henry Simmons) are keeping

SPOILERS AHEAD

Sif (Jaimie Alexander) first shows up emerging from the ocean on a beach in Portgual. A young man tries to help her, but she tosses him 100 feet into the air. She has amnesia but only knows one thing: she must find Kava.

Back at the base, Skye and May train together, and Skye actually gets the upper hand. May is not satisfied, because she thinks Skye needs to embrace her emotions and nerves on her terms, and use them in battle. If she did that, she’d probably split the state in two. That’s due to her new earth-shaking powers that she’s trying to hide.

Meanwhile, Morse and Hunter (Nick Blood) seem to get really friendly these days, and he seems to be willing to stay as a SHIELD agent. That’s a concern for Mack, and whatever he and Morse are planning. Maybe that’s why he is a little reluctant to be a field agent for Coulson, although he claims violence is not his thing.

The crew finds Sif, who knows she is from Asgard, but not much else. She doesn’t even know the name “Thor”, although it does make her smile. (Did she watch him on SNL last weekend?)  She says she has the “mind of a lost adolescent”, especially when she says “Shut. Up.” when Coulson says she met Odin once. She’s still a tough warrior, though. Through online videos, Sif sees she was battling someone who was just as tough as she is. We later find out the guy has blue skin and something he calls a truncheon. He is a Kree called Vin-Tak (Eddie McClintock), and for some reason they were battling each other. He’s concerned that something has changed on Earth and he needs to find it.

Morse tries to corner Vin-Tak, but he pushes her away. Skye pulls a gun at him, but her tremor powers emerge knocking a shelf on to her, giving him a chance to leave. She tells Fitz what happened, but he tells her to keep quiet about it.

Meanwhile, Mack warns Morse not to get too close to Hunter because he may threaten whatever they have planned. Mack doesn’t object to her reconciling with Hunter, just not now. He’s even worried Lance could kill her if he knew the truth. They do say they’re not HYDRA, which means maybe it’s a rogue part of SHIELD that they support. Mack just hopes in the end they’ll be in a world where they can all be friends. That hope will end sooner than they think.

Coulson and May figure out Vin-Tak is headed to Chaves, Portugal which means “keys” in english. “Kava” also means “keys” in Cree, which is why Sif was saying it to herself. Vin-Tak tries to drag away a coffin in Chaves when SHIELD gets the drop on him. He explains he used the truncheon to take Sif’s memory so she couldn’t interfere.  Apparently the Kree and Asgard are not friends. Once he does restore her memory, she figures out she had to find Vin-Tak because he was headed to Earth. He came down because he sensed Terrigenesis had occurred, namely someone using the Diviner last December. He also says the Kree used Terrigenesis to make soldiers centuries ago, until that was stopped. He’s worried once the Kree find out that Diviners are being used again, making Inhumans, they may be back. The coffin Vin-Tak found is supposed to have six Diviners, enough to make an army. They’re missing, although we know Gordon (the guy with no eyes at the end of “What They Become”) has one.

It’s not long before Vin-Tak and Sif find out Skye has experienced Terrigenesis, and her new powers make her a major threat. It’s already caused a major rift between Fitz and Simmons, and is about to break open the base. May hides Skye in Vault D, the same vault that housed Ward a while ago. May tries to calm Skye down, but Sif is determined to get through. Vin-Tak also wants to eliminate Skye, but Hunter shooting a big gun and Morse using the truncheon stop him. Skye stops the tremors the only way possible:  shooting herself with an Icer. Sif is impressed by Skye’s decision.

Still, Skye’s new powers have caused a lot of damage in the base, and among the crew. Fitz argues with the others over how they would have handled Skye, and says that they need to protect her. Mack finally says, “We’re the ones that need protection from her…” and she’s right there to hear it. The unity within the agents is at an all-time low, and Skye’s thinking she’s lost her family. Maybe she’s also thinking Cal Zabo, her dad, was right when he said no one would understand what she has become, and that she would be feared.

Sif escorts new amnesiac Vin-Tak back to his home world. Before that, she discusses the agents’ decision to protect Skye, and she can see why it’s logical. Still, she warns Coulson “I’m saying there are tides in the universe that you cannot swim against.” May agrees with her, realizing this may be all about fate. Coulson bring brought back from the dead gave him DNA that included plans to find an alien city, and that was followed by Skye joining them. She also remembers what Lumley said, “Wherever Skye goes, Death follows.”

Finally, Hunter confronts Mack saying that he knows about the flash drive, and wants to know who is “backup.” Mack answers him with a sleeper hold. It’s more damage within the SHIELD crew, and it’s not because of Skye.

This episode showed that SHIELD is being threatened by forces more destructive than HYDRA: fear and mistrust. It existed between Sif against Vin-Tak, Mack and Morse against Hunter, Fitz against Simmons and now everyone against Skye. It’ll take a lot to repair the cracks within SHIELD, if they can be repaired. The revelation about Terrigenesis, and its connection with the Kree, could be the first signs of how the upcoming Inhumans movie will develop. Whether Skye will be part of it remains to be seen.

Whether SHIELD will still exist also remains to be seen, when Morse and Mack finally admit their true mission.

Next week:  May turns to her ex-husband to help Skye, while Zabo (Kyle MacLachlan) organizes his own group of super-villains to battle SHIELD.

Entertainment Weekly has more on tonight’s episode, and how it’ll lead to a major story this season. Comic Book Resources also has an interview with Chloe Bennett on what Skye may face after becoming an Inhuman, including a possible love interest.

Marvel’s Agents of SHIELD airs on Tuesdays at 9/8c on ABC. Watch full episodes on ABC.Go.com, Hulu Plus and On Demand (check with your local cable provider.