I won't rewrite the whole show, every scene on the show that isn't rewrite happened as in S1,S2 or S3.
That chapter takes place between E5 and E6. The morning of Black Star day.
Black Star day is marked on the calendar for 12/03, it's the date on Campbell's clock when Grace confronts him. The date on that same clock the morning after the police raid read 11/22. So I believe that Grace stayed 12 days after the guns were discovered.
He won't stay, he never does. She'll propose tea and he'll say he needs to go. For now, she is watching him, trying to memorize every detail of his face, so calm and serene in that moment. So closed and stern much of the time. So scared and demp 3 nights ago when he woke her up by his scream.
He has come to her 4 times since the night of the police raid. She has stayed for those moments. She should have been gone as soon as the guns were found. Her mission was finished. She had resigned and it would have been safer for her to leave the town immediately.
But she couldn't.
She wasn't sure that Campbell would keep his part of the deal and leave the town as he had found it. Leave Tommy be.
She was right to doubt.
After that night it was so hard to go. She had decided that she would escape on the black star day. When everyone, especially Tommy, would be busy far from town.
Today is the day.
Tommy will conquer the betting stand and dethrone Kimber as the first gang of Birmingham.
By the time they come back from the races, she'll be gone.
She won't see the betrayal in his eyes. She won't see the life, the warmth that has shone through them, only for her, gone and replaced by that coldness and calculation so familiar to everyone around him.
She has felt the warmth and the passion of Tommy Shelby...That first night…
Then by the early morning, he ran out, telling her he would be back a few days later.
He came back, not at the Garrison, where she was anxiously watching everyone who foot step through the doors. He came to her place, after her shift. Shy and respectful once again.
She offered tea, which this time again they forgot about in seconds.
But that night, they drank some tea, after… while talking about his family… and hers. She told him about her mother and her brothers, how they died. About her father, how much she admired him even though he was a stern man. About his death, but not the way he died…
They crossed paths at the Garrison in the day that followed and he asked to come to her place late, because they still wanted to remain discreet.
She couldn't leave, she needed to engrave those memories, to remind herself how full life could be.
That life will end today. She'll go to London and then to New-York.
She'll be on her own. Still a mission to accomplish for the Crown, but no more revengeful drive.
She hopes that Campbell and Tommy will forget about her and let her live her life without seeking revenge for what both will see as a betrayal.
She knows she'll never feel for anyone the way she feels for Tommy. But there's hope that she'll find someone with whom she'll share affection and respect.