Ok, it really depends on the game. Which game do you want to know about? Mother, Mother 2/Earthbound or Mother 3?

In mother, Maria and George went missing. However, only George returned. He was doing experiments to study the mysterious powers of PSI. You must collect eight melodies. Later, you find Maria. She had suffered from amnesia. The final boss Giegue, was raised by George and Maria. His race told him to attack humans. However, Giegue does not want to turn on those who raised him. You must play the eight melodes, which Maria used to play to him as a kid. The melodies get him to remember all of his fond memories, and turns back to Maria. In Mother 2/Earthbound, a Meteorite crashes by your house. An alien warns you that Giygas will take over in ten years, and all is devistation. You must go to eight "Your Sanctuary" locations, where your soul will be amplified by the earth. Each location holds a memory that reminds you of your younger self. The eight melodies must be played to gain access to the final boss, and your friend, Pokey (Porky in the japanese version) has turned against you and tricked you into battling Giygas, though he is a good alien. You must pray. Everyone prays for your safety and grants you the strength to fight. In Mother 3, You are visiting your family in Tazmily village. However, Flint, your dad, chose to stay home. Hpowever, a group of pig-masked people kill your mom, who is trying to protect you and your brother. After hearing of her death, Flint snaps and ends up in prison. You, Lucas, run away. Your brother Claus disappears. After traveling in search of your brother, Pokey/Porky informs you that he has killed him and brainwashed him. You must kill your own brother, but you can't. Your body won't let you kill your own brother. Then, after you think it can't get sadder, you hear your mom's voice. She tells Claus to stop. Her voice does not get to Clause. Finally, you must see your brother die in front of you. Lucas saw his mother die. His brother disappeared and was killed. Finally, he saw his Brother die (fans say he commited suicide). The motto is "No crying until the end"