It involves Chuck Bass - sort of a jerk, a player (has slept with most of Manhattan's female population), and already an alcoholic and into drugs at the age of 17 - and Blair Waldorf - a bitchy, scheming, plotting "Queen B" of her school, Constance Billard, who dreams of a happy ending and gaining access to her dream school, Yale.
During the beginning of Season 1, they don't have much romantic moments; they are only known to have been friends since they were younger, along with Serena van der Woodsen and Nate Archibald, although they broke away as they grew up. However, in the episode Victor/Victrola, everything changes when Blair gives her virginity to Chuck after getting drunk because she was upset about her breakup with longtime boyfriend, Nate. Chuck, who's never had a steady relationship and only a steady stream of one night stands, immediately becomes infatuated with her. Later, at her seventeenth birthday, he confesses that he hasn't been able to sleep, and feels "butterflies" in his stomach. They sleep together again and they continue a secret relationship for a few days.
During season one, there's a lot of drama, mostly including Blair and Nate getting back together, and Chuck's agonizing jealousy of the couple. Finally, when they are brought together again to protect Serena from an old enemy, Georgina Sparks, they realize their feelings for each other, which Chuck acknowledges at his father's wedding, when he gives a best man speech, inspired in the moment by Blair. They later dance, where Chuck tells her she "doesn't belong with Nate. Never have, never will." She replies with "You don't belong with anyone." and they kiss, becoming a couple. However, their happiness doesn't last long when Chuck is reluctant to give up the old him - the womanizing, boozing part - and never comes to meet her in Tuscany as he promises, leaving her alone for the rest of the summer.
The majority of season two includes Chuck and his pursuit of Blair, trying to get her back from his previous mistake. It's revealed that Chuck hasn't been able to sleep with anyone since her, and decides he needs to have Blair one more time, and that would allow him to sleep with someone else again, once his other needs are satisfied. This plan isn't successful, as Blair is determined to have a relationship with her new boyfriend, Lord Marcus Beaton, a British lord, who just might be able to give her her dream of becoming a princess. However, they break up when it's revealed Marcus has been having an affair with his stepmother. Although, Chuck is still unwillingly celibate.
Later in the season, it's revealed that Chuck cannot tell Blair "I love you", even if he is very much in love with her. They play games with each other, both trying to get the other to say those "three words, eight letters" but neither succeeding. When the Upper East Side finds out Chuck's father has died in a tragic limo accident, Chuck is devastated and in an attempt to help him Blair admits she loves him. He bitterly tells her "Well, that's too bad" and leaves her heartbroken. While he is gone, his uncle, Jack, seduces a depressed Blair.
When he returns, he seeks the comfort of Blair, finally allowing himself to break down about his father's death. They fall asleep, but when Blair wakes up, she finds a note, saying "Sorry for everything. You deserve much better. Don't come looking for me. - Chuck."
The rest of the season involving the two focuses on the "games" they play, and their attempts to seduce the other and try to get the other to succumb.
In the season finale, Gossip Girl accuses Blair of being a "weakling" and Chuck a "coward" about their inability to admit their love to each other. Blair, determined to prove Gossip Girl wrong, confesses to Chuck that she loves him and their love "consumes her". She tries to get him to say it back to her, but GG sends out another blast, this time revealing Blair's one night stand with Jack Bass, and Chuck insists he doesn't love her.
However, at the very end of the episode, Blair finds Chuck waiting outside of his limo, with gifts and flowers. He explains he was in Europe to buy her these things, and that he couldn't run away from her anymore. He admits that he loves her, they kiss, and there is the season 2 happy ending.
Chuck and Blair is love. <3
Chuck: Please don't leave with him.
Blair: Why? Give me a reason and "I'm Chuck Bass" doesn't count.
Chuck: 'Cause you don't want to.
Blair: Not good enough.
Chuck: 'Cause I don't want you to.
Blair: That's not enough.
Chuck: What else is there?
Blair: The true reason, I should stay right where I am and not get in the car. Three words. Eight letters. Say it...and I'm yours.
Chuck: I... I ...
Blair: Thank you. That's all I needed to hear.
Chuck: I'm Chuck Bass! No one cares.
Blair: I do! Don't you understand? I'll always be here! I don't want you going anywhere. I couldn't bear it. So whatever you wanna do to yourself, please don't do that to me. Please.
Chuck: I'm sorry.
Blair: It's okay.
Chuck: You were right. I was a coward running away again, but everywhere I went, you caught up with me. So I had to come back.
Blair: I want to believe you, but I can't. You hurt me too many times.
Chuck: You can believe me this time.
Blair: Oh. That's it?
Chuck: I love you too.
Blair: But can you say it twice? No i'm serious, say it twice!
Chuck: I love you, I love you, that's three, four, I love you.