But as Shonda Rhimes tells TV Guide, she and the other Grey's writers knew about Maggie's existence since the show started 10 years ago.
"What's interesting for us is that Maggie has been a piece of story that we have had and known about… honestly since the beginning," Shonda says. "That was one of the original pieces of story since the beginning with the Richard-Ellis dynamic, but it's been a piece of story since Season 4 when we talked about introducing her. We were going, 'When's the right time?!' and we were holding it forever."
The 44-year-old producer also reveals that we viewers could have been left in the dark about Ellis's other daughter forever: "I kept going, 'Not this season. It's not right.' After a while, I was like, 'We're never going to introduce it. The show is going to end without it.' Every time I thought the show was going to end, I was like, 'Ah!' Then we hit this moment in time and I remember walking into the writers' room and going, 'You guys, it's time.' And all the writers knew exactly what I was talking about."
And lest you think that this is an egregious example of "retcon" — i.e. retroactively changing a backstory to serve a current plotline — Shonda says that there have been hints about Maggie throughout the series so far.
"It's not a thing where we're going in and backdating," she asserts. "It's literally always been layered into the storyline. We can literally go back and pull out moments of the show, and I'm excited to do so and point out where it was talked about. It's not going to be so hard to do."
Is your curiosity piqued? Do you have any ideas where those telling moments were? Give us your thoughts in the comments below.
Grey's Anatomy Season 11 premieres on Thursday, September 25, at 8 p.m. ET on ABC.