For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Ivanhoe starts with a huge power imbalance. As the recent conquerors of England, the Normans have all the power, and they lord it over the Saxons as much as they can. Saxon Cedric of Rotherwood is so angry at this injustice that he disinherits his only son, Wilfred of Ivanhoe, for joining Norman King Richard I on his Crusade to Palestine, and he engages his lovely ward Rowena to Athelstane, the last surviving descendant of the Saxon royal family. It's impossible to see at this point how Scott is going to smooth over the differences between the Normans and the Saxons, which is one of the main threads of Ivanhoe's plot.
Act II
Act II is where the plot is as far from its resolution as possible. Things look rocky between the Saxons and the Normans during the tournament at Ashby-de-la-Zouche, where English representatives Ivanhoe, Robert Locksley, and the Black Knight all do well against the Norman jerks. But the worst moment for Norman-Saxon relations is the whole episode of the battle at Torquilstone. Norman knight Maurice de Bracy kidnaps Cedric, Rowena, and Athelstane to force Rowena to marry him. The English outlaws of Sherwood Forest band together under the leadership of the Black Knight and Robert Locksley to destroy the Normans' castle. The only Norman to escape relatively unharmed is Brian de Bois-Guilbert, but he also takes a captive: Ivanhoe's savior, Rebecca. The battle at Torquilstone makes the bad blood between Ivanhoe and Bois-Guilbert that much worse.
Act III
A lot of the plot of Ivanhoe is Saxons-vs.-Normans, but we know that there are good Normans (King Richard I for one) as well as bad ones. Scott has to bring the Saxons and the Normans together by the end of the book to give some hope of an English future that will blend the two cultures (since, historically speaking, that's what actually happened). Cedric warms up to King Richard after he realizes that he helped rescue the Saxons and bring down Torquilstone in Act II. Cedric also gives up on the Saxon king idea when his ideal candidate, Athelstane, swears loyalty to King Richard and refuses to marry Rowena. By the end, Cedric forgives Ivanhoe, Ivanhoe and Rowena get married, King Richard helps Ivanhoe prosper, and they all live happily ever after (with some exceptions – see "What's Up With the Ending?" for more). By the end, the strong separation between the Normans and the Saxons is breaking down.