Hello!
Naina & Meghna are sisters. The story begins with Meghna's marriage getting fixed to Kunal Chauhan, who's also her love interest. Karan is Kunal's brother and NaiRan meet for the first time while organising a date for their Di & Bhai respectively. Naina realises Karan is extremely reserved and doesn't like if anyone touches him. On the marriage day, the Chauhans are insulted by the Ladki waale, who end up cursing that it's a sin to give their beti in such a household. Kunal's father, to save his honour, keeps a condition that Meghna will only be accepted as their bahu if Karan marries Naina.
So, they end up getting hitched against their wishes to some extent. Karan refuses to accept her and asks her to leave on their wedding night.
Later, it's revealed that Karan has a skin disease which is contagious, hence he didn't want Naina to marry him. But Naina vowes to heal him with her love and patience.