Because the group was slacking in practice, due to weak competition at the upcoming sectionals, Will created a boys vs. girls competition in order to motivate the team. The winning side would choose what song to sing at sectionals.
Finn was having trouble getting into it, though, because he was a stressed out mess. The pressure of Quinn's baby, playing quarterback and being a major part of the glee club was too much for him. He wasn't sleeping. He couldn't concentrate. As a result, he went to see the school nurse: Terri.
Here's how she got that job: Sue visited Terri at home and told her about Will and Emma. She made it sound like there was actually an affair going on because Sue wanted to destroy her rival. Terri, therefore, convinced the principal to make her the school nurse, despite any experience whatsoever. Now, she'd be able to keep an eye on Will all day.
Because she had no idea what she was doing, of course, Terri wasn't sure what to do with Finn. She ended up giving him over-the-counter decongestant (we'll call it Vitamin D going forward), which is actually no longer sold. But it worked wonders for Finn, as he led the boys on a tremendous "mash up" of "It's My Life" and "Confessions." Will loved it, while Rachel was dismayed at how well the guys did.
Kurt sold out their Vitamin D-inspired rendition, though, which initially angered Rachel (she compared Finn to Alex Rodriguez!), but she and the girls soon gave in: Terri also gave them Vitamin D for their performance.
While all this was going on, Ken was realizing that Emma wasn't so into him (she cried when his elbow once brushed against her breasts), so Terri inspired him to come up with a plan: propose! Ken actually did so, which took Emma by surprise.
Back to the glee club: the girls also kicked major booty with their performance. How would Emma (who Will named as judge) choose a winner?!? She wouldn't have to.
Terri eventually told Will and the principal about the Vitamin D, getting herself fired, getting Will mad at her and also getting Will in trouble because he was too focused on winning and wasn't even paying close attention to his kids.
How would the principal solve this problem? He named Sue as co-head of the glee club! That wasn't the only piece of bad news for Will. The episode ended with Emma telling him she accepted Ken's proposal.
Source: TV Fanatic