The official press release for Supernatural season 13 episode 21.
 
SUPERNATURAL
“Beat the Devil” — (8:00-9:00 p.m. ET) (Content Rating TBD) (HDTV)


THE LONG JOURNEY HOME – Sam (Jared Padalecki), Dean (Jensen Ackles), Castiel (Misha Collins) and Gabriel (guest star Richard Speight, Jr.) must work together if they have any hope of bringing Mary (guest star Samantha Smith) and Jack (Alexander Calvert) home. Meanwhile, Rowena’s (guest star Ruth Connell) encounter with Lucifer (Mark Pellegrino) may alter the outcome of the journey for one of our heroes. Phil Sgriccia directed the episode written by Robert Berens (#1321). Original airdate 5/3/2018.

Source:

The CW

 
So Mary and Jack still haven't been rescued. Gabriel is working with Sam, Dean and Cas.  Rowena encounters Lucifer. Is she still trying to kill him? Whose journey will be affected by this meeting? Let us know in the comments or in the discussion thread for episode 21.