'Supergirl': SPOILER Returns in "For Good"

Supergirl's newest installment saw plenty of revelations, unexpected surprises, and the return of a familiar villain.

Spoilers for tonight's episode of Supergirl, "For Good", below!

Tonight's episode followed Lena Luthor (Katie McGrath) dealing with her conflict with Morgan Edge (Adrian Pasdar). After Morgan accused Lena of sabotaging his car, Lena denied it -- and was poisoned shortly after.

Lena tried to track down exactly who poisoned her, and found her answer: her mother, Lillian (Brenda Strong). Lillian revealed that she was orchestrating the whole thing to get her daughter back to the side of evil and kill Morgan in the process.

Lillian threatened to kill Morgan at a benefit (using some familiar pieces of technology), but Lena, Kara, and Mon-El (Chris Wood) protected him. One of Lillian's drones then transformed into Lex Luthor's warsuit, which covered Lillian as she fought with Kara and Mon-El.

Ultimately, the pair defeated Lillian, who was taken to the hospital and then to jail.

This return took fans by surprise, as Lillian has been missing in action since the end of Supergirl's second season. Lillian ran the endlessly shady Project Cadmus, which she used to enact Operation Exodus, a plan to rid the country of its registered aliens. Kara Danvers/Supergirl (Melissa Benoist) ultimately stopped the plan, but it was unclear exactly where Lillian ended up.

In the newest episode, it's clear that Lillian's motivations have shifted slightly, with her being less concerned with her anti-alien sentiment, and more with whether her daughter was living up to her full potential. And even though Lillian seems to be headed to jail once again, it's safe to assume that she will eventually return to wreak more sorts of havoc on Supergirl.

Supergirl airs Mondays at 8/7c on The CW.