Time and Space Door Marks

"Do you still want to go back?" On his eighteenth birthday, Yan Kongli received an old book. This is the only line on the title page. He thought it was a prank until cloud patterns began to appear on his wrist, and a pure white space and a falling figure appeared repeatedly in his dream. Until he saw a "person" that only he could see huddled in a flower bed in the night market. Until a throwing knife was nailed to his feet, and a man in uniform said to him: "That thing on your wrist is not a tattoo." Song Kongli thought the book was a gift. Later he discovered that the book was the key——