You don’t know us too well, do you?
Season One: Buffy is a new sophmore at Sunnydale High. Sunnydale is built on top of a Hellmouth, which is a portal directly to Hell. Her nemesis is The Master, an ancient vampire who tried to open the Hellmouth back in the '30s, but the ceremony was interrupted by an earthquake which swallowed the corrupt cathedral and imprisoned The Master. At the end of the season, Buffy was killed by the Master, but resusitated by Xander. She then staked the Master and sealed the Hellmouth. Buffy also meets and falls in love with Angel, a vampire who used to be especially nasty until a group of gypsys cursed him with a soul, thereby making him feel guilty over all the evil he’d done.
Season Two: Spike and Drucilla, a particularly tough pair of vampires and former compatriots of Angel’s, show up in Sunnydale. Drucilla is suffering from some sort of sickness that can only be cured by sacrficing Angel (Dru’s sire). Although Buffy interrupts their ritual and saves Angel, Dru cured. Spike, however, is crippled, and spends most of the rest of the season in a wheelchair. Later, Buffy and Angel finally consumate their relationship, Angel’s love for Buffy gives him a moment of “perfect happiness.” For a second, he stops feeling guilty over all the evil he’s committed, and his curse is invalidated, he loses his soul, and he becomes completely evil. He hooks up with Spike and Dru, and kills Jenny Calender, Giles’ girlfriend and descendent of the Gypsy tribe that originally cursed Angel. He also unearths the demon Acathla, who, if awakened, will destroy the Earth. Meanwhile Willow, a burgeoning witch, finds a spell Jenny Calendar developed to return Angel’s soul. Spike reveals that he has been faking his injury. Jealous about over Angel’s relationship with Drucilla, he betrays them both by helping Buffy. Angel starts the ritual to awake Acathla, which can only be stopped by killing Angel. Buffy and Angel fight, she’s about to kill him, when Willow finishes her spell and restores Angel’s soul. Regardless, Buffy has to kill him; it’s the only way to close the portal. She does so, than leaves her friends and family to run away to LA.
Season Three: Buffy returns to Sunnydale and reconciles with her friends and family. It turns out that, when Buffy died in the first season, a new Slayer was called. Faith, the new Slayer in question, shows up in Sunnydale. Initally Buffy’s friend, she accidentally kills an innocent human, and can’t cope with the guilt. Eventually, she betrays Buffy and joins sides with Mayor Willikins, a powerful warlock who is planning on turning himself into a demon, and needs a massive sacrifice to complete the transformation. He plans to use the graduating class at Sunnydale high as that sacrifice. Meanwhile, somehow Angel escapes from the hell dimension Buffy sent him to at the end of the last season. He has his soul back, but centuries in the hell dimension (time flows differently there) have reduced him to an animalistic state. Buffy, without the help of Giles or her friends (who all hate Angel) nurses him back to sanity. Eventually, Faith posions Angel, and the only cure is the blood of a Slayer. Buffy tries to kill Faith to get the blood, but only succeeds in putting her into a coma. Instead, Angel feeds on Buffy. During graduation, Buffy and co. enlist the entire graduating class to fight the Mayor, and finally kill him by luring him (now in the form of a giant snake) into the library, which is filled with explosives. Angel and Buffy realize that they can’t be together, and Angel departs for LA and his own TV show.
Season Four: Buffy goes to college. She meets a student named Riley Finn, whom every Buffy fan worth his or her salt despises. Willow, who has been dating Oz, a guitarist and werewolf, is heartbroken when Oz leaves Sunnydale to find a cure for his condition (the werewolf part, not the guitarist part). She cheers up quite a bit when she meets Tara, and realizes that she’s a lesbian. Riley, as it turns out, is actually an experimental super-soldier working for a special army task force dedicated to controlling supernatural menaces called The Iniative. His CO is Maggie Walsh, who is posing as a philosophy teacher at UC Sunnydale, and their HQ is an enormous facility underneath one of the Frat houses. The Iniative captures Spike, who has returned to Sunnydale after being dumped by Drucilla, and installs a computer chip in his head that causes extreme pain whenever he harms a human being, but not, as it turns out, when he harms demons. Professor Walsh becomes jealous of Buffy and Riley’s relationship, and worries that Buffy might interfere with some of her less savory experiments, so she tries to bump Buffy off. The attempt fails, and Maggie is killed by one of her experiments: Adam, a demon-human-cyborg hybrid. Riley has a crisis of loyalty between his unit and Buffy, but chooses Buffy. Spike, rejected by the demon community because of his inability to hurt humans, teams up with the Scoobies against Adam and the Iniative, although he ends up selling out the Scoobies, without much success. Adam becomes a messianic figure for the demon inhabitants of Sunnydale, and convinces them to allow the Initative to capture dozens and dozens of demons. He lures Buffy and pals into the Iniative, then locks the doors and releases all the demons, hoping that the threeway demon/human/Slayer battle will cause an enormous slaughter, giving him plenty of spare parts to build himself an army. Buffy uses a powerful spell that gives her the combined powers of the entire Scooby gang, and she kills Adam.
Season Five: Suddenly and inexplicably, Buffy has a sister: Dawn. No one on the show is surprised by this mysterious sibling. It is later revealed that Dawn is actually The Key, a powerful source of magic energy that can unlock the barriers between dimensions. A hell-God named Glory wants her for a sacrifice so she can return to her home dimension. A group of monks used a powerful ritual to bind Dawn into a human form, and altered the memories of everyone in Sunnydale, including Dawn’s, so that no one would realize that she’s only been around for a few weeks. Meanwhile, Spike realizes that he’s in love with Buffy, and tells her that Riley has been cheating on Buffy with a vampire hooker, for no particularly good reason. Riley departs the show to fight demons in Central America. Good riddance. Buffy’s mom is diagnosed with a brain tumor, which is successfully removed, but she is later killed by an anyeurism. Spike, pining for Buffy, approaches a guy named Warren, who is a genius at robotics, to build him a BuffyBot to have sex with, but the bot is destroyed when Glory kidnaps Spike and tortures him to find out where the key is. Spike doesn’t spill the beans. Glory, the Hell-God, needs to feed on the sanity of humans to maintain her own sanity in the mortal realm. Also, she shares her body with Ben, an innocent human, and they frequently morph back and forth. While trying to locate the Key, she feeds on Tara’s (Willow’s gf) sanity, making her insane. Buffy realizes she can’t fight Glory, and tries to flee, but fails. Dawn is kidnapped by Glory and nearly sacrificed. The Scoobies show up, and Buffy beats the crap out of Glory, using a variety of magical artifacts that they’ve accumulated over the season. Finally, she morphs back into Ben, who promises never to return to Sunnydale. Buffy leaves him to save Dawn, but Giles comes upon the helpless human and kills him with his bare hands, ending the threat of Glory forever. However, the ritual has already started, and reality is being torn apart. Buffy sacrifices herself to close the dimensional rift opened by Glory.
Season Six: The Scoobies are using the repaired BuffyBot to keep the local demons from realizing they have free reign over Sunnydale, while they work to ressurect Buffy. It works, but it turns out that Buffy had been in heaven, and she’s massively depressed about being alive again. To make herself feel better, she gets involved in a very abusive relationship with Spike (Buffy is the abuser). Meanwhile, Warren (the robot guy from last season) allies with Jonathan (a minor recurring character since the first season) and Andrew (nobody special) to take over Sunnydale. All three of them are geeks of the highest calibre, although only Warren is really, really evil. Most of their schemes are more annoying than actually dangerous. As all this is going on, Willow becomes dangerously addicted to magic, and almost kills Dawn. Her actions force Tara (sanity restored during the climactic fight with Glory) to leave her. Eventually, they reconcile, just in time for Warren to flip out and try to shoot Buffy. He wounds her, but a stray shot hits and kills Tara. Insane with grief, Willow starts using magic again and tortures Warren to death. When the Scoobies try to intervene, she turns on them and lays a major smackdown on the lot of them. Determined to end all suffering in the world, Willow tries to destroy it, but Xander manages to talk her out of it.
That, more or less, brings you up to speed with this season. Hope that helps!