In an official video, Gamble admitted that the "lighter" instalment is a reaction to the previous episode's "heavy [and] difficult" scenes.
"We were talking about how to follow up Rufus's death, and the idea of doing a quirky episode about 'What would happen if the Titanic never sank' came up," she explained. "Balthazar (Sebastian Roche) went back in time and un-sunk the Titanic, so all of those people who were supposed to die, didn't die.
"Mysteriously now, accidents are befalling these people. Some being wants to kill the descendants of the Titanic [survivors]."
She claimed that 'My Heart Will Go On' will explore the "butterfly effect" of Balthazar's time meddling.
"The fun and challenging thing about breaking this episode was figuring out what would be the same and what would be different," she said.
Gamble also confirmed that the deceased character Ellen Harvelle (Samantha Ferris) will return in the episode.
"We bring her back in what we hope is this unexpected way," she teased.
Watch Sera Gamble discuss tonight's episode of Supernatural below:
Supernatural returns to The CW tonight at 9/8c.