Short answers, pulled from the story.
Nader Shah began his invasion of India on the 6th of November 1738 when he resumed his march through Indian territory. He had previously consolidated power in Iran and defeated Afghan tribes to secure his western flank before entering Mughal domains.
Financial crisis in Persia due to declining exports and famine drove Nader Shah to seek spoils from India to fund his army. Contemporary sources also suggest Mughal vassals plotting against their suzerain were behind the invasion while political fragmentation made the subcontinent an alluring target for a conqueror short on finance.
Indians killed 3,000 Afsharid troops during riots that broke out on the night of the 21st of March 1739 after rumors spread about Nader Shah's assassination. This event triggered the qatl-e-aam massacre ordered by Nader Shah on the morning of the 22nd of March.
Nader Shah seized the Koh-i-Noor and Darya-i-Noor diamonds among other treasures when he entered Delhi on the 20th of March 1739. These jewels now form part of British and Iranian National Jewels respectively following the sack of the city.
The Battle of Karnal resulted in a decisive victory for Nader Shah over the Mughal forces led by Muhammad Shah. The Persian army annihilated the Mughal defenses allowing Nader to enter Delhi and demand surrender keys from the emperor.