[Warning: The following contains spoilers for Thursday's episode of A Million Little Things. Read at your own risk!]

It's been a minute since A Million Little Things fans have been consumed with a mystery like, "Who is Barbara Morgan?" Well, it looks like it is once again time to put on our questioning hats. Thursday's episode of A Million Little Things kicked off a new mystery that will last into the next season, according to executive producer DJ Nash.

Eddie's (David Giuntoli) sister, Lindsay (Sprague Grayden), arrived in Thursday's episode to reconnect with her brother and bond over their sobriety. While there were some hiccups in their visit, the two finally got on some solid ground. That is, until the end of the night when Lindsay asked Eddie if he remembered the lakehouse they used to visit as kids. It was obvious Lindsay was asking for a reason, but Eddie couldn't figure it out. Something definitely went down there, and we'll be finding out more in the coming episodes.

Meanwhile, Maggie (Allison Miller) got a last-minute placement in the Oxford fellowship and could be heading off to England very soon. Rome (Romany Malco) and Regina (Christina Moses) had a terrifying encounter with Eve (Ebboney J. Wilson) and her ex, but they were able to get the couple separated and Eve safe.

In the next episode, the group of friends will reflect on the loss of Jon (Ron Livingston) on the first anniversary of his death. TV Guide talked to Nash about this episode's revelations and what to expect from next week's "Dreams" episode.

David Giuntoli and Ron Livingston, <em>A Million Little Things</em>David Giuntoli and Ron Livingston, A Million Little Things

How big of a part is this lakehouse mystery going to play in the final three episodes of the season?
DJ Nash:
Huge. This is the mystery we're following for the rest of the season and into next season. Obviously, something happened in their past, and it appears as though Lindsay may have started drinking because of it — or certainly, it was a catalyst for her drinking to try to forget what happened. It's something that because Eddie was drinking, he wasn't lucid. He doesn't totally remember...We are tracking him uncovering more about something that happened in their past, and we'll see the effect that has on both of them.

This is coming up right as Katherine quit her job and Eddie found out that Dakota is using. Do you ever feel like you are putting too much on these people at one time?
Nash:
What I'll say is that I think when Eddie told Katherine, "I'm in a good place, I can handle this," by design, he meant it, and he was. Now, we are making it so he's not going to be in a good place. It's the perfect storm of unfortunate events that happened to this couple, just when they're in a great place. Fans of our show, I've heard them say like, "Oh, Eddie and Katherine are in such a good place. It's not as exciting." It's about to get exciting.

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Looking ahead we have this big Jon episode coming up next week. The photos from that episode are already out. Will he help Eddie figure out some things about this lakehouse?
Nash:
Yes. I'm going to confirm something. Congratulations.

Who might be in the most sensitive place when this anniversary comes around?
Nash:
People have asked me, "Oh, so is Ron [Livingston] still on the show? Is he still part of the show?" I think he's on the show in the way someone you lose remains in your life, which is, it's not that you don't think about them. You think about them, probably less often...The idea of having him back for the anniversary of his passing seemed really appropriate. It's an episode where the characters have each had dreams about him over the course of the last year. Well, most of them have, I should say...Jon's death was this catalyst for them all to make a change and now the anniversary of his death is a barometer for them to measure how much or how little they've changed.

Maggie is over the moon that she got this Oxford fellowship. What do you say about the group's reaction when they find out she's heading off to England?
Nash:
I think there's a group's reaction and then there's Gary's reaction. I think the group is really torn. A year ago, they lost their North Star. As they tried to move on, ironically, at his funeral, they added an important member [to the group] who literally sits in his chair at a family dinner. I think for all of them, they'll be coping with that possibility of her leaving in different ways. Rome had done her first therapy. Delilah certainly opened up. There's a place that [Maggie] has in this friend group that's been really important. For Gary, sometimes it's just a matter of losing something that makes you appreciate it.

If there was any impetus for a grand romantic gesture, like a John Hughes at the airport kind of moment — this would be it.
Nash:
It appears as though nothing can get in their way. I'd love it if you quote me on that.

Gary also had a bit of chemistry with Darcy in this episode, which is exciting for fans. Will we be seeing more of her in the upcoming episodes?
Nash:
I mean, it's unbelievable. I will say, I don't know that we've had a guest star with as much anticipation. I'm glad because we're going to deliver. She's great in the show. She's awesome. You're seeing a lot more Darcy, yes.

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Roman and Regina had a pretty exciting episode. Are they going to have a minute to breathe or are they still working out a lot of things with Eve after they got her to the safe house?
Nash:
Obviously, it appears as though the threat of Derrick is over, at least in the short term. Now, they can go back to the idea of the adoptive parents to Eve's baby. That said, the normal boundaries that you ought to have when you are the adoptive parents of a child have been blurred, if not broken. As they try to move forward, they're going to have to try to find the balance of what's the right level to have Eve in their lives.

What are you most excited about for fans to see in these final three episodes of the season?
Nash:
I think we have done some episodes that have really shocked people. The most shocking episodes have yet have to come.

A Million Little Things continues Thursdays at 10/9c on ABC.