- "What happened to Sara?"
"It's called time drift. The same thing you and Mr. Palmer experienced, albeit on a much lesser scale. If one stays for too long in a foreign time, they begin forgetting their past and who they really are."
- —Rip explains to Kendra how Sara has forgotten her past.
Ray, Sara and Kendra are shocked after they watch the Waverider fly off without them, leaving them stranded in the '50s. After waiting 2 years for their teammates to come back, the three realize they must move on with their lives. Ray and Kendra bond as a couple but Sara decides to return to the League of Assassins and Ra's al Ghul.
- One of Ray's students is William H. Gates, Sr., the father of Microsoft co-founder Bill Gates.
- This episode shares the same name of a book, game, and movie franchise titled Left Behind.
- This episode shares the same name of Arrow's "Left Behind". This name can be explained by the presence of Arrow's third season's main antagonist, Ra's al Ghul or by the fact that Sara Lance, Kendra Saunders and Ray Palmer are left behind by the Waverider.
- Matt Nable makes his return as Ra's al Ghul.
- Sara's time in the League appears to be a bootstrap paradox.
- Ray questioned why Rip didn't attempt to go back to when the Waverider was hijacked, so to spare him, Sara and Kendra two years in the 1960 where technology was more primitive. Rip offers a cop-out excuse.
- Rip challenging Ra's to a trial by combat creates a continuity error. According to Ra's in Arrow's The Climb, which takes place in 2014, the last time someone challenged him to a trial by combat was 67 years prior to 2014, meaning that he was challenged in 1947. However, in this episode, it was shown that Ra's was challenged in 1960. It is possible that Ra's doesn't count 1960 because he wasn't the one who fought.
- Rip explains that staying in a foreign time for too long causes a time traveler's memory to start warping. This would be further explained in The Magnificent Eight.
- The med bay of the Waverider can recreate missing limbs; Snart froze and smashed his hand to get free of some handcuffs, and Gideon later recreates his hand for him.
- In the promo, it's been confirmed that it's been two years since the last episode for Ray, Sara and Kendra.
- This episode marks the first appearance of a child named Talia. Talia al Ghul is famous known in the comics as the Daughter of Ra's al Ghul as well as the Wife of Batman (Bruce Wayne) and the mother of Damian Wayne. Talia will later have a major role in the fifth season of Arrow.
- The Shadow Record mentions a member of the League called Fadir Nasser, who in the comics is known as the White Ghost.
- In the comics, the White Ghost's real name is Dusan al Ghul, the only son of Ra's al Ghul.
Preparation ran from December 7 until December 15, 2015. The first part of shooting ran from December 16 until December 19, 2015. The second part of shooting ran from January 3 until January 12, 2016.
- ↑ DC TV: 'Bug-EyedBandit', 'Eddie Thawne', And 'Ra's Al Ghul' Feature In ARROW, THE FLASH & LOT Synopses - Comic Book Movie
- ↑ Cite error: Invalid
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|DC's Legends of Tomorrow season 1|
|"Pilot, Part 1" • "Pilot, Part 2" • "Blood Ties" • "White Knights" • "Fail-Safe" • "Star City 2046" • "Marooned" • "Night of the Hawk" • "Left Behind" • "Progeny" • "The Magnificent Eight" • "Last Refuge" • "Leviathan" • "River of Time" • "Destiny" • "Legendary"|