Britney Spears makes her "Glee" debut next week -- but preview clips of the cast singing her biggest hits leaked online today!
Check out Heather Morris' Brittany singing "Slave 4 U" and "Me Against the Music," Lea Michele doing "One More Time," the boys taking on "Stronger" and the whole gang tackling "Toxic."
The "Britney/Brittany" episode airs Tuesday.
