Maleficent's story continues in the upcoming episode of "Once Upon A Time" titled "Mother." It is still unknown if more of the remaining villain trio will be revealed here as the title is understood to pertain to the dangerous diva, but the synopsis for the episode, provided by The Futon Critic, suggests a different take on the story. 

Emma, who is rapidly slipping into her darker self, comes back to Storybrooke in the hopes of reuniting Lily and her mother, Maleficent. This is after their altercation in the previous episode that instigated Lily's evil plan involving Mary Margaret and David. 

This episode will also have a lot going on for Regina. The Queen and Robin are finding it unimaginably difficult to deal with wicked witch Zelena's pregnancy. Viewers will also be able to take a peek into Regina's past involving her family. 

In a flashback, Regina's mother Cora comes back from Wonderland in time for the anniversary of Daniel's murder, hoping her daughter would forgive her. She then gives her word to Regina that she will help her "find true love," a promise Regina is still suspicious about. 

Gold's health is deteriorating, leaving Isaac no choice but to find another ally who could help him get a hold of the magic ink to modify the past. It remains to be seen where his search will lead him but rewriting the past has become more difficult for him now. 

The season is also expected to go on with Emma and Lily's seemingly unending feud. Just like her mother, Lily turns out to be another baddie as well. And fans predict that Emma is also on her way to becoming sinister and they expect that by the end of the finale, the savior will fully transition into a powerful foe. 

"Once Upon A Time" season 4 episode 21 "Mother" will air on May 3.