“Far from the Tree” is episode three of season three of Supergirl.
Alex and Kara are discussing the wedding shower when J’onn arrives. He needs to go away for a few days and needs them to manage the DEO. J’onn explains that, at the end of the previous episode, “Triggers”, M’gann contacted him and said she needed help. Kara says that’s a suicide mission for the last Green Martian and will go with.
She asks whether they could call Winn and use the transmat; J’onn says they need a gate on the other side. They have to go the old-fashioned way. He opens a garage, revealing an old convertible. Supergirl assumes that there’s a lift beneath it. No; they’re taking the convertible to Mars. J’onn comes from a race of shapeshifters; their tech shapeshifts too. It’s a spaceship.
Eliza, Alex and Maggie are eating and Eliza asks about there being no childhood pics of Maggie. Maggie hasn’t talked to her parents since she was 14, when her father dumped her at her aunt’s.
Supergirl and J’onn arrive on Mars and head underground. He opens a door and they meet M’gann. She introduces some White Martians; Till’all and N’keyy. They raided a White Martian base where there was a prison camp for Green Martians still in operation. J’onn isn’t the only Green Martian left. There’s another. M’gann brought J’onn here because he wouldn’t believe unless he saw. She activates a screen. The other Green is M’yrnn J’onzz. J’onn’s father.
M’yrnn is a high religious leader and not even the Whites would kill him. It seems that he knows the location of the Staff of Kolar. J’onn says that’s a myth but M’gann says it’s real and the Whites know where it is. It’s a psychic weapon and it could be used to track down every resistance fighter and kill them. They believe M’yrnn knows where the Whites are digging, but he’s closed his mind. They need J’onn to convince him to help.
J’onn goes to see his father, who doesn’t believe it’s him. For one thing, M’yrnn’s son would never have fled. Afterwards, Till’all says they have no time. They need to invoke the Rite of Tigar. Take the location of the Staff by force. Which could kill M’yrnn. Supergirl wants another choice.
On Earth, Alex and Maggie talk about Maggie’s family; Alex suggests inviting them to the shower. And Maggie ends up ringing her father. On Mars, Till’all isn’t willing to wait for M’yrnn to cooperate, and M’yrnn somehow needs convincing that J’onn really is his son.