Richard and Alexis will investigate an in-flight murder case of a Air Marshal.Facebook/Castle

ABC crime series "Castle" will once again focus on Richard Castle (Nathan Fillion) and Alexis Castle's (Molly Quinn) relationship in Season 7 episode 21, titled "In Plane Sight", which is set to air on Monday, 27 April, at 10 pm.

The official synopsis and promotional photos of the next sequel hint at a complicated investigation by the father and daughter with the help of Kate Beckett (Stana Katic).

As the two travel to London, their plane's Air Marshal is found murdered and they race against time to find the killer before he carries out his fateful plan, states the description of the episode. 

The investigation team includes Martha Rodgers (Susan Sullivan), NYPD Captain Victoria Gates (Penny Johnson Jerald), Medical Examiner Lanie Parish (Tamala Jones), NYPD Detectives Javier Esposito (Jon Huertas), and Kevin Ryan (Seamus Dever).

However, Fillion's character and his on-screen ladylove may not share screen space together as they will be investigating the murder case from different places.

While the former mystery novelist and his daughter will do an in-flight investigation, Katic's character will make plans to identify the murderer who's about to move on to the next phase of a dangerous plan from a remote location. So it remains to be seen how the three will solve the mystery and arrest the culprit on 27 April.

Guest stars in the episode include Inbar Lavi as Farrah Darwaza, Nick Greene as Aiman Haddad, Paula Newsome as Debbie Parker ,and Josh Dean as Jim Kogut.

Meanwhile, "Castle" fans can tune into ABC this Monday, 20 April, at 10pm , to watch Season 7 episode 20, titled "Sleeper", which will be live-streamed here.