The manga starts with our main character holding his sister (who is dying) on the ground. One of the villain of the story, beside the two, and the reason the sister is like this now is because of the villain beside our main character. It turns out that the villain wanted the main character's ring which has power however as the villain who right now is very powerful and kills our main character.
I believe our main character swallows the ring and is then killed. He dies vowing to kill the villain. He wakes up and it turns out that he has gone back in time to right before the world-changing event happens. Now with his knowledge of the future, and how to get stronger faster than anyone else, his plan is to kill the person who killed him and his sister before the villain becomes to much to handle.
As the outbreak begins, chaos ensues and his first objective is to find and protect his sister. While on the journey to find his sister he meets a "Traders" as they are called in this world. Anyone can use these entities called "Traders" but you must have the crystals that you obtain from the zombie like creatures you kill. Our main character ring allows him to have a storage type system like an inventory in a game and also allows him to get discounts with the traders. The ring has more powers I believe but I do not remember what they are.
After trading with the "Trader" and heading to save his sister our main character find two of the most powerful people in his old world who right now are weak and he takes them under his wing because he knows he much have a powerful team to be able to survive in this world and he is determined to do things right this time.