She seems more grounded than usual, telling Will that she's taking medication and seeing a therapist. The two end up having sex.
At school, meanwhile, New Directions need a teacher. Kurt ends up asking the woman who is subbing for Will in Spanish class: Holly Holliday. She's a free spirit who lets the kids do almost anything and who bashes Will's taste in music. She wants the group to update its set list as it decides what to sing for Sectionals.
Such an attitude endears Holly to the group, even to Rachel, who admits that she's always wanted to sing a fun dance number - so Holly joins her on stage for a number from "Chicago."
But after Mercedes gets in trouble for taking Holly's advice (stuffing tater tots into Sue's tail pipe in protest of them being banned), Holly breaks down, visits Will and realizes she's too flaky to be an effective teacher. She has commitment problems and fears passing these along to the kids. Terri actually walks in on these two chatting, freaks out a bit and is told by Will to leave, that the night before was a mistake.
Will ends up coming back to work because Sue realizes Holly is even more danger than her other nemesis, and is embraced by his students.
Source: TV Fanatic