The swamp invasion
Shrek is happy alone until Lord Farquaad dumps every fairy-tale creature in the kingdom on his swamp. To get his land back, he agrees to rescue Princess Fiona from the Dragon's tower and hand her over to the lord.
With Donkey stuck to his side, Shrek pulls it off and falls for Fiona along the way. Overhearing half of a conversation, he thinks she's rejecting him for being an ogre. He comes around in time, crashes the wedding and learns her secret: Fiona is an ogre too. They get married.