Here is your ten-point cheat sheet on this big story:
A furious Amarinder Singh, facing rebellion and in-house attacks for months, had told Sonia Gandhi this morning he had put up with enough humiliation. "This kind of humiliation is enough, this is happening for the third time. I can't continue to stay in the party with this kind of humiliation," Mr Singh, aka "Captain", has reportedly told the Congress president.
Amarinder Singh's son Raninder Singh made it official with a tweet in which he said: "...must go now as I am proud to accompany my father to Raj Bhawan when he submits his resignation as CM of Punjab and leads us as head of our family into a new beginning et all."
Mr Singh had fought the party's order to resign, which raises the possibility of a split in the Congress just months before the Punjab election.
Mr Singh had also called a meeting of MLAs to rally his loyalists, as numbers came into play long before the 117-member Punjab assembly votes early next year. Fifteen of the Congress's 80 MLAs, including four ministers, attended the meeting.
Sources say over 50 MLAs had written to Sonia Gandhi demanding that Amarinder Singh be replaced as Chief Minister, forcing the party to announce late last night an emergency meeting of MLAs.
The names of three leaders are in circulation to replace him - former Punjab Congress chiefs Sunil Jakhar and Pratap Singh Bajwa, and Beant Singh's grandson Ravneet Singh Bittu.
Hours earlier, Sunil Jakhar had all but announced that Mr Singh was on his way out. "Kudos to Rahul Gandhi for adopting Alexandrian solution to this Punjabi version of Gordian knot. Surprisingly, this bold leadership decision to resolve Punjab Congress imbroglio has not only enthralled congress workers but has sent shudders down the spines of Akalis," Mr Jakhar tweeted.
The Punjab crisis has escalated dramatically over the Chief Minister's festering feud with Navjot Singh Sidhu. In July, despite the Chief Minister's fierce resistance, the party appointed Navjot Sidhu its Punjab chief, but the acrimony stayed barely below the surface.
The truce started unravelling with a row over advisers appointed by Mr Sidhu and their controversial statements, which Mr Singh publicly condemned.
Last month, four ministers and around two dozen party legislators raised fresh complaints against Amarinder Singh, telling the leadership that they had no faith in his ability to honour poll promises.