Jenova is not the calamity she's portrayed as. She was simply an explorer who crash landed on a certain planet, and unfortunately the local populous reacted badly to her natural physiology. Now things are spiraling out of control, and she's trapped, fading in and out of consciousness for thousands of years.
Until one day, a man obsessed finds her, takes from her, and creates a son for her. She did not birth him, but he is her's all the same. She pines for him and wishes for her son.
Then one day a certain chocobo headed blonde child gets a little too close to a certain reactor, and Jenova reacts, reaching out to him in her grief, instinct calling for a son, and poor little Cloud answers.
Now being the only one who understands the Calamity for what she truly is, Cloud sets off to Midgar with a bit better of an edge than he might have normally, to join Soldier and meet up with his big brother, get him to meet mother, and hopefully clear the air, without a certain bastard scientist getting in the way…
And perhaps Mother will be able to save her eldest son's best friends while she's at it. Assuming she isn't driven mad by Hojo's tinkering and the Planet's natural immune response trying to burn her from the inside out…