This was a complicated and very uneven episode, and I'm not even sure which one of this week's plot was the 'A' plot.
Let's start with the most interesting thread. The episode opens in Austria, 1945, where Nazi scientist 'Daniel Whtiehall' aka Reinhardt (Reed Diamond) tests the alien doorstop on various prisoners from the Chinese village where they found it. Most have apparently turned to stone. One prisoner, a woman we will call Obviously Skye's Mom (Dichen Lachman), makes the doorstop sparkle.
Sadly for Reinhardt, and happily for Obviously Skye's Mom and Western Civilization, the war ends and Reinhardt is captured and interrogated by Peggy Carter (Hayley Atwell, back again to sell her forthcoming show, Agents Of SHIELD Rises). Reinhart tells Peg a tale of blue aliens coming to save or maybe conquer the planet (or maybe destroy it, according to Kyle MacLachlan later in the episode), and offers to trade his freedom for everything he knows.
Peg don't play like that. She locks Reinhardt away in a prison called The Rat (was "the Raft" taken?), where he can grow old and be forgotten -- until HYDRA agent Alexander Pierce (Robert Redford, not present) arranges his release.
Restored to his Austrian castle, Reinhardt discovers that Obviously Skye's Mom is still alive and hasn't aged, so he chops her up and steals her youth to make himself young again, and reinvents himself as Daniel Whitehall. Achievement unlocked: Origin story!
Elsewhere in the plots; Kyle MacLachlan (Kyle MacLachlan) explains to Whitehall that the doorstop "chooses" worthy people, and that these people will be able to enter into a temple in the Mystery City, at which point the doorstop will do something extraordinary. No need to explain further! That's more than enough plot for now! La la la!
At good guy (ish) headquarters, the team goes digging through Peggy Carter's files and learns that Whitehall and Reinhardt are the same person. Morse (Adrianne Palicki) interrogates captured HYDRA henchman Bakshi (Simon Kassianides), and she interrogates him so hard that he takes cyanide. Yes, they checked for cyanide, but he had extra cyanide. Morse and Hunter (Nick Blood) then argue about this so hard that they have ex sex.
Coulson (Clark Gregg) takes the other half of the team on a convoluted mission to hack a satellite so that they can map the planet and find the lost city. HYDRA has the same idea at the same time and shows up in the same place, naturally, and in the ensuing firefight, Tripp (B.J. Britt) is shot.
Thankfully Kyle MacLachlan is on hand to save/endanger his life, and uses the opportunity to have a fun-but-gratuitous villain/hero face-to-face with Coulson in which MacLachlan asserts his parental rights and gets super cray cray when Coulson calls Skye "Skye.'
Agent Trip survives, and Coulson successfully maps the planet and gets a match on the city that I'm trying very hard not to just call 'Attilan'.
In addition to all these inter-connected threads, there's also the matter of boring ol' Grant Ward (Brett Dalton) abducting his brother, Senator Christian Ward (Tim DeKay) at their old family home so he can force Christian to admit that he was the one who manipulated Grant into almost killing their youngest brother Tommy (not shown) in a well. Christian confesses, and apologizes, and says he wanted Tommy dead because their mother loved him the most.
Later, Grant offers his services to Whitehall, and Whitehall commends Grant for arranging the death of Christian and the Ward parents in an apparent murder-suicide-fire. Bye Tim DeKay! Hope you had fun!
In the kicker, Kyle MacLachlan returns to HYDRA HQ to take his place in the new Whitehall-Ward-MacLachlan triumvirate of evil.
But a flashback reveals that a much younger Kyle MacLachlan long ago swore revenge on Whitehall for killing Obviously Skye's Mom, whom, it turns out... may have been Skye's mom.