A plot point which seems to belong to Plot A belongs to Plot B also/instead. The plotlines aren't actually related, so we're not
Working The Same Case, but a point or character introduced as related to one line ends up being important to the other.
Compare to
Two Lines, No Waiting,
Working The Same Case,
Strange Minds Think Alike. If Plot B is sufficiently minor as to make you wonder "What's it doing here?", then the character is
Chekhov's Gunman.
Examples
Literature
- Peter Lovesey's The House Sitter: Peter Diamond investigates the murder of a profiler who was tracking a serial killer called The Mariner. Maybe The Mariner killed her, but maybe it was her jealous boyfriend. Well, she was killed in a somewhat random act, not by The Mariner. And by the way, The Mariner is the boyfriend.
- Robert Van Gulik's The Lacquer Screen had several separate storylines. The killer in the A plot was a character in the B plot; the character in the B plot was introduced in the C plot, and the killer in the C plot was from the D plot.
Live-Action TV
- Friends episode "The One With the Videotape": the B plot involves Monica and Chandler being given fake names by a couple they met. To illustrate that people do this, Joey introduces himself as "Ken Adams." The A plot involves a seductive story Rachel tells Ross, unaware Joey already told him the story. When questioned, Rachel sputters that she heard the story from a friend, who heard it from a guy named Ken Adams.
- The later seasons of Seinfeld do this often, many times a mixture of this and Strange Minds Think Alike?
- All versions of CSI: Crime Scene Investigation beat this trope to death in multiple episodes, such as finding a fingerprint of a suspect in the Plot B crime at the scene of the Plot A events.
- Babylon 5 did this a bunch. For example, "The Quality of Mercy," with Laura's clinic and the murderer Mueller.
- How I Met Your Mother episode "The Naked Man" has plot A with Barney and Ted trying out a new tactic called the The Naked Man (no points for guessing what they do) and plot B is Lily writing a list of 50 reasons to have sex, which she completes after trying The Naked Man tactic (2 out of 3 times people!)
- An episode of Castle had two teams of detectives competing (and betting) on who was going to solve their respective cases first, they were forced to a draw when the two cases were related.
Other
