The initial beginnings of this relationship starts in "Love's Labours Lost in Space", where Leela, out of pity, sleeps with Zapp Brannigan. Interestingly, before this event, Leela was actually a fan of Brannigan, but when Brannigan tries to win her back, she becomes furious with him.
On the other hand, Zapp becomes very cocky after having slept with Leela, and would so, on further occasions, constantly believe he still has a chance. The relationship (and the event) was referenced numerous times by Bender in the first and second season. After that it purely becomes a joke appearing less and less.
In the comics, Zapp occasionally loses interest in Leela, to which Leela actually becomes upset.
In "In-A-Gadda-Da-Leela", Leela and Zapp were lost together at Lo'ihi Island. Zapp made Leela believe they were on another planet and that they were out of water so she accedes to make love. Leela then realized Zapp was lying, but they later were forced to make love by V-GINY to save Earth.
In "Zapp Dingbat", Zapp starts dating Leela's mother, Munda. While Leela doesn't show any jealousy she is convinced that Zapp is using Munda to get to her. She attempts to seduce him to prove he's only interested in her but the plan fails. Though Zapp appears in later episodes, this is the last time him and Leela have any interaction.
Bender: [After seeing Zapp on TV.] Hey, look, Leela, it's that idiotic windbag you slept with.
Leela: The Earth is under attack. Can't we just forget about that?
Bender: Evidently not.
- Episodes where their actions between these characters based on the event, as well as references by other characters.
- "Love's Labours Lost in Space" (1ACV04)
- "A Flight to Remember" (1ACV10)
- "When Aliens Attack" (1ACV12)
- "I Second that Emotion" (2ACV01)
- "Brannigan, Begin Again" (2ACV02)
- "War Is the H-Word" (2ACV17)
- "Amazon Women in the Mood" (3ACV01)
- "Kif Gets Knocked Up a Notch" (4ACV01)
- Into the Wild Green Yonder
- "In-A-Gadda-Da-Leela" (6ACV02)
- "Zapp Dingbat" (7ACV05)