Get ready to welcome back your favorite mystery novelist ... turned private investigator? That’s right, “Castle” returns to ABC Monday, Sept. 21, with its Season 8 premiere. Although the Season 7 finale left Rick Castle (Nathan Fillion) and Kate Beckett (Stana Katic) on a good note, not all will be well when things resume.

Check out 8 things to know about Season 8 before Monday's highly anticipated premiere at 10 p.m. EDT:

1. Episode 1 Synopsis

Episode 1, titled “XY,” will reveal Beckett’s career choice: captain! The couple will be “adjusting to Beckett’s career change” when a call sets off a serious of events that result in the NYPD detective going missing -- and mercenaries on her trail. Fans will see Castle’s side of the story as he searches for his beloved wife. But don’t fret. Viewers will get to see Beckett’s side of the situation in a follow-up episode. Episode 2, "XX," will air Sept. 28.

2. New Showrunners

Showrunner David Amann stepped down after one season, but series writers Terence Paul Winter and Alexi Hawley are tackling the job in Season 8.

3. Meet Hayley Shipton

There is a new investigator in town. “The Neighbors” alum Toks Olagundoye is joining Season 8 as an investigator named Hayley Shipton. The twist is that she’s not working with the police, which means she has the opportunity to break rules and have fun. Expect her to work alongside Castle now that Beckett has her important new gig as captain; however, no romance will be blooming between them. Olagundoye made it clear in an interview with TVLine that her new character is not being introduced to get between Castle and Beckett.

Castle Season 8 -1 Toks Olagundoye has joined "Castle" Season 8 as a private investigator named Hayley Shipton. Photo: ABC

4. A New Partner

Hayley Shipton is not going to be the only one working alongside Castle. His daughter, Alexis, appears to be following in his footsteps. A new promo photo for Season 8 teases that Castle’s private investigator venture will turn into a family business.

5. Baby News

There is a baby on the way! Kevin (Seamus Dever) and Jenny (Juliana Dever) will be welcoming another kid in Season 8. Surprisingly this won’t just shake things up in the Ryan household. Co-showrunner Winter dished to TV Guide that the new baby will complicate things at work for Kevin and his partner Javier Esposito (Jon Huertas).

6. A Season 7 Storyline Returns

Fans remember when Castle went missing in Season 7. Although the ABC series has moved on, that storyline will reemerge in Season 8. Executive producer Hawley revealed to EW that the show will wrap up some of those mysteries in an “unexpected and exciting way.”

7. Guest Stars

Expect to see new faces in Season 8. EW reports that “The O.C.” alum Kelly Rowan will play the “dean of a prestigious university” in an upcoming episode. Her character, Carla Feller, will be distraught after the death of a student.

8. Sneak Peek Videos

Ahead of Monday’s premiere, check out these sneak peek videos from episode 1:

The "Castle" Season 8 premiere airs on ABC on Monday, Sept. 21 at 10 p.m. EDT.