From reddit: Ok, beginning right after Aki´s death. Makima takes Denji home and he´s suffering so much guilt for killing Aki that he says he doesn´t want to think anymore, just obey her(thus becoming her dog in a metaphoric sense). It was then that Makima reveals that when Denji first died and Pochita became his heart a contract between him and Pochita was made, basically Pochita would give Denji´s his powers in exchange for him showing him his dreams. After Denji’s friends are all killed, and he surrendered his free will to Makima, AND he kills Power the contract was broken(as Denji gave up on all his dreams). It was then that Pochita resurfaced, but on his true form, the four armed goliath thing with the intestine scarf(Makima later reveals that Pochita´s dog form was just the result of him battling the four horsemen and the weapon devils and becoming really damaged in the process, thus becoming that cute dog devil that Denji found).
Makima then says that Pochita was actually really feared in hell due to his ability of erasing devils and their concepts by eating them( for example, eating the Aids devil would erase the concept of aids in the real world as well) and she really wanted his powers for herself in order to erase all bad concepts in the world and, in her words, make it a better place. Thing is, Pochita was fused with Denji when she found him, so she made Denji´s life miserable enough for him to break his contract with Pochita and ressurect him.
Now that Pochita is back on his true form he seems to still want to fulfill Denji´s wishes like eating hamburgers and going on a date. Makima however, wants to defeat Pochita in order to control him(her control abilities only work on those she deems inferior to her) but she gets bodied hard, well, at least until the latest chapter, in which is revealed that Denji´s defeating the gun devil was actually her plan to make everyone admire Chainsaw Man. Since devils are powered by fear, admiration actually fucks them up, so Pochita/Chainsaw Man became really weak and Makima finally defeated him
From me: After chainsawman is killed, Pochita begs Power to help save Denji (Pochita can only do this because Denji drank Power’s blood). Power agrees and is able to resurrect for a short time while she takes Denji’s body to safety. There Power and Denji form a contract, Denji gets Power’s powers but he has to go find the Blood Devil in Hell and turn it into Power again after he kills Makima. This brings Denji back and Kishibe is waiting for him. Denji has a talk with Kobeni which leads him to understanding that his dreams are still possible and not all is hopeless. He goes to fight Makima. Makima kicks his ass, but this was all part of Denji’s plan. The chainsawman Makima was fighting was actually Pochita not Denji (Denji can remove his heart/pochita and it can turn into the chainsaw-devil *we are only just learning about this ability*), and Denji formulated this plan after realizing Makima has never even seen Denji or acknowledged him, just Pochita and Pochita’s ability. Since Makima can only identify beings by smell (which we also just now learn), this allows Denji to sneak up on her and kill her again, because she only knew Pochita’s scent. Denji finally eats Makima, utilizing the Chainsaw-devil’s ability to erase that being from existence and memory. However, this doesn’t erase the control-devil, just Makima. Kishibe finds the reincarnation of the control-devil, Nayuta, and entrusts her to Denji. Kishibe hopes that by leaving her with Denji instead of the government, the control-devil will be able to realize its dreams of having a family. Which it tried to accomplish as Makima, but in a twisted way.
Part 1 ends. Now Part 2 will most likely focus on Denji bringing Power back but finding the Blood-devil in hell.