Doctor Who and his wife Song Jiang have opposite timelines. The first time the doctor saw her was the last time she saw a doctor, but I still don't understand.

Song Jiang's timeline goes upstream along the doctor's timeline.

When the tenth doctor first met her, they were dealing with The Weeping Angels on the Byzantine. After solving The Weeping Angels, Sung River was taken away. The doctor asked Song Jiang what he had done wrong. Song Jiang said that she killed a person, that is, a doctor. She was killed by silent training at birth. This is Song Jiang's original timeline. Song Jiang didn't know the doctor at that time. She killed the doctor, but at that time the doctor already knew that she knew her, and everyone told Song Jiang. The doctor is the most important person to her. Because Song Jiang was born to Amy, Amy spent her wedding night in Tadis. At that time, there was a whirlpool of time, so Song Jiang had the ability of time Lord. When she killed the doctor and learned that the doctor was the most important person to her, she gave all her rebirth strength to the doctor and brought him back to life.

Later, she became more and more familiar with the doctor. Her timeline didn't end until Dr. 12 knew her best and loved her most. 12 After the doctor had a good time, Song Jiang took a task and went to the library to save people. There she met the tenth doctor, a doctor who didn't know her at all, and she knew that this was her last time. She chose to sacrifice herself, trapped in the terminal forever and became a data ghost.

At this point, the timeline has formed a ring. On her honeymoon with the twelfth doctor, the doctor gave her a sonic screwdriver. Later, when Song Jiang took this sonic screwdriver to the library and sacrificed himself, the sonic screwdriver of Dr. Ten at that time did not have the function of uploading data ghosts. But when the tenth doctor knew that Song Jiang was his future wife, he understood. He immediately picked up the sonic screwdriver brought by Song Jiang and found that it really has the function of uploading data ghosts. He uploaded Song Jiang to the terminal and asked her to live a data ghost life.