The season four premiere was told out of order with certain plot lines running forward in time, while others running backwards.  Our recap is set in forward time because we'll, that's how our watches run:

Brock awakens in bandages from the explosion of the Monarch's car in an OSI plane and overhears some doctors talking about brainwashing him.   Though it turns out they were actually talking about Sgt Hatred, he mistakes it for him and grabs a near by doctor.  They try to explain why he's on the plane and why he has H.E.L.P.ER's head in his chest, but it's too late, Brock jumps out the plane.  Brock returns back to the Venture compound, knocks out the OSI agents watching the boys, and gives Dean his wrist communicator watch and takes a bag with him to go.  He tells Dean not to let anyone know he was there but to give Dean his best.  Brock heads to the doctor who performed Hunter's gender change to have him remove H.E.L.P.ER's head and has it mailed back to the Venture compound.  Brock then goes to stay with Steve Summers and Sasquatch while he recovers from his surgery and ends up an overweight, hairy version of himself.  He heads to exact revenge on Molotov and Hunter but finds out someone is already there doing the same thing... SPHINX.  Brock ends up being shot by Molotov and a team from SPHINX rescue him and he wakes up imprisoned by them.  Hunter, sans his boob implants, reveals it was all double crossing and he's been apartof SPHINX the whole time.  Wait, what?

Back in the Venture compound, Sgt. Hatred, now brain-washed, is the boys' new bodyguard and he's struggling to get the guys to trust him.  Henchman 21, meanwhile, comes by visit Dr. Venture and wants to pay him to clone Henchman 24 and tries to give him a priceless family heirloom, the first Marvel comic, as payment.  Dr. Venture dismisses the comic book as trash and gives it to Hank who destroys the comic book throughout the episode and every time he does its value it displayed in the top corner, indicated time passing.  However, it turns out number 21 isn't the only one who needs cloning because a group of Nazis kidnap Rusty and try and force him to clone a dog that shares Hitler's DNA into a human Hitler.  When 21 returns later, Venture tells him he can't because of the Nazis, so 21 goes to see Dr. Orpheus about raising him from the dead.  However, Orpheus admits he's only a necromancer for the cool name, not because he can actually raise the dead. 

Dean decides to take the Hitler dog as his own which he names Giant Boy Detective, and everyone is telling him he needs to kill Hitler using a golden dagger given to him by Orpheus, but he refuses.  Srgt. Hatred and Hank try and perform a rescue mission to save Dr. Venture from the Nazis, but it's actually Dr. Orpheus who tries to protect Dean and the Hitler dog from H.E.L.P.Er (whose head is now attached to the spider) with a burst of electricity that kills the Nazis.  After the credits, Brock is back in shape and wearing a SPHINX uniform and rescues Hank from the Hitler dog and stabs it with the golden dagger.

Wow, now that's a season premiere!