Made a video.
i also take the Black Nightshade Flower collectible.
This was taken from my walkthrough of the game..
This is chapter 14 - Blood Ties
This is probably the hardest trophy to get in the game..
You must get in the door first before Nicholas.
If Nicholas enters first he close the door and you won't get the trophy.
I also grab the Black Nightshade flower without restarting the checkpoint..
Hope this helps.;)
A Plague Tale: Innocence - Captain Sidekick Trophy with Black Nightshade Flower