Summer Smith (304-X) was the daughter of Beth Smith, the step-daughter of Jerry Smith, the older sister of Morty Smith, and the granddaughter of a Rick Sanchez, whom she leaves after he destroys the world. In their new dimension, she was part of the Resistance against dictator Morty.
Several years prior, Beth brought Summer and Morty from their original dimension to Dimension 304-X to escape the world that their Rick had destroyed. They stole his Portal gun to leave him, but it was not too weak to use. Once in the new, Morty started growing smarter, but gained an obsession with ridding the multiverse of Ricks. He created a monster to do that, but this drove him and his sister to fight. Beth left Morty and Beth to join the Resistance, after Morty had seized power as dictator. Once she saw Rick and Morty from C-132 she saw her way out of that dimension.
Summer saves Rick and Morty, from Dimension C-132, after they had traveled to her dimension and was cornered in an alley by a hitman looking to kill Rick. She sneaks with her back to the hideout of The Resistance where she proceeds to show them around. She then explains to them how her Morty is a cruel dictator and gets into an argument with Rick before explaining more about her Morty's backstory. She meets Rick again later and she says he came to her dimension to help them, not just hide. Their conversation is interrupted when Jerry holds Morty (C-132) hostage with a gun and ends up accidentally shooting him. Jerry then activates a bomb and blows up the entire Resistance hideout.
Summer somehow survives the explosion and brings a still alive Morty to her appartment. While she has lost hope, Morty still has one more idea. They head out, sneaking past Walkers, to the Dictator Morty's base. They shoot their way through guards before they reach a room with a Portal gun, where Summer tells Morty (C-132) about her past and reason for helping him. She replaces the batteries in portal gun, making it able to transport only two people. they are then attacked by Morty (304-X) in a Walker, whom Summer notes speaks a lot like a Rick at this point. This infuriates Morty (304-X), but he exits the Walker to prove he is not like a Rick. He accuses Summer of being the reason why he is this way, but gets told by Morty(C-132) how it's the universe's fault instead. Summer and Morty (304-X) reconcile just before they are both crushed to death when Jerry enters with a huge armoured vehicle.