Doctor Who Boss and Karen Gillan on River Song, How Her Parents Met and Hitler

Only in the world of Doctor Who can the time traveler you've seen as a maternal figure end up being your future, grown-up daughter.

That's the rather absurd reality that Amy Pond (Karen Gillan) faced about the mysterious River Song (Alex Kingston) in the June midseason finale and will continue to cope with when the show returns with the episode "Let's Kill Hitler" on Saturday at 9/8c on BBC America. "After that big revelation, it is going to change the dynamic between everyone," Gillan tells TVGuide.com.

Showrunner Steven Moffat admits that he didn't think of the time-bending twist when River first appeared two seasons ago, but once he decided he wanted Kinsgston back, he cast Gillan as the new companion to the 11th Doctor (Matt Smith) with that possibility in mind. "I used the name Pond for that reason, to create a link," he explains. "It was Plan A for a long time ... There is the new companion; she's getting married the next day. My favorite dirty joke in Doctor Who is the title of Episode 13 in Series 5, 'The Big Bang,' the night when River began. I laughed a lot when I thought of that."

Despite the flip-flopped ages, Amy and River have much more in common than their ginger-tinged hair. "I think that the moment that you actually see them together, it makes sense, how similar they are in certain aspects," Moffat says. "That sort of devil-may-care attitude, how naughty they are. Either of them, without the presence of The Doctor, would just be wicked."

"What we are going to really explore is that relationship between Amy and River," adds Gillan. "Now, it is kind of flipped. Amy has these maternal instincts towards her daughter obviously. Me and Alex Kingston have so much fun playing around with that."

Check out just how maternal Amy feels about her baby Melody Pond, aka River Song, in this short "prequel" to "Let's Kill Hitler":

The paternal instinct will also kick in for Amy's husband, the much beleaguered Rory (Arthur Darvill). "That's a rather complex one for him isn't it?," Moffat says. "He's always had this slight jealousy of The Doctor and Amy's slightly too-close relationship. ... Now he's got the fact that his daughter is clearly madly in love with this man. It works out in interesting ways, not quite what you'd expect."

In exploring Amy and Rory's newfound parenthood, the show will also flash back to when they grew up in the same small town of Leadworth. "We are going to see a little bit into Amy and Rory's past and how they were friends as kids and how they got together as teenagers," Gillan says. "My cousin, Caitlin [Blackwood], plays the young, young version of Amy Pond, and then I play the kind of teenage one."

She doesn't reveal, however, if young Rory (Ezekiel Wigglesworth) has always been in love with Amy or if their relationship had to develop into affection over time. "We see the beginning of something," Gillan teases. "That Rory breaks my heart. He is so cute."

With an episode titled "Let's Kill Hitler," at some point, The Doctor will encounter the notorious leader of the Nazi Party. Moffat warns, "It's actually not quite the centerpiece of the episode, but it is one of the great time-travel conundrums: If you've got a time machine, why haven't you killed [Hitler]?

"It's a story that makes you think it's one thing, then it's quite another," he continues. "I think it's one of the funniest ones that we've done. It's a great, boisterous, fun episode.  It's one of my absolute favorites."

In this preview of the new Doctor Who episodes, we see The Doctor's run-in with the Fuhrer (Albert Welling) and proof of Rory's heroism:

"Let's Kill Hitler" airs Saturday at 9/8c on BBC America, followed by the special, Doctor Who: Best of the Companions, at 9:50/8:50c.