Steve was MIA and Natasha got news that he was dead. She wrote in a diary often and wrote about how she felt towards him. Until he comes back and finds the diary.

After the events on Age of Ultron, the Avengers parted their ways and Steve Rogers was set to lead the new Avengers team with Natasha Romanoff. However, as their partnership grew, Steve wanted Natasha to be more than just a partner , while Natasha's still persistent on keeping her secrets to herself. It wasn't until a mission gone wrong that they had to learn to get to know each other in a rather... unusual way.