Main character has the ability to steal things their owner considers valueless, or if he meets special circumstances. Pre-regression the regression user used his ability to steal things to load the main character down with curses. The regression user had feigned any liking for the MC as he just wanted to use him. Same with all of the female characters around the regression user. Regression user had unlocked the ability to regress others with him and was going to regress the female characters with him.

Right as the regression was going to trigger the MC stole it as the regression user was utterly tired having regressed an unknown number of times before reaching this point. MC gets regressed along with the female characters. Regression user has not made a re-appearance in the story since, but two of the female characters who regressed have played a role in one arc.

What follows is MC powering up largely through things he knew or guessed from his first life. He loses the stolen regression, so no do-overs for him post regression (good author removing an OP ability for a logical reason).

MC befriends some erosion beings (monsters that can develop sentience from the world ending phenomenon known as erosion), including an immortal one who becomes his master. He takes revenge on a family member, though he tries to give him some a chance to redeem himself.

With this family member, we learn how skill stealing works on things that are valued by their owner. He has to fufill conditions that can be subjective, such as being more powerful than an opponent, being seen as a rival, etc. This is also a clever way the author makes an otherwise OP skill, more interesting.

It has yet to be determined what happens to someone after a skill is stolen if they are alive. Curse stealing has no negative impact and has been used once so far to enhance the MC by using the curse properties to suppress downsides of powerful other abilities. It seems in this arc we are in now the MC will be stealing his second skill.