Really, he does throw a tantrum. For the entire episode.

So, after timeskip, Nowaki returns to Japan from studying abroad in Amerikkka. He went to study pediatrics, but the problem is, failed to inform Hiroki ahead of time. Instead, the only notice that Hiroki receives from Nowaki is a postcard sent not long before Nowaki's return. This is the last straw for Hiroki, who decides he wants to break up with Nowaki after six years. (Wait, six years?)

Well, Nowaki heads straight for Hiroki's apartment, and the latter lets him in. Hiroki maintains a consistently cold demeanor to Nowaki, though, so Nowaki decides to leave. Hiroki stops him, and smooth as ever, Nowaki seduces Hiroki into having hot steamy woohoo. (And by smooth I mean blatantly molesting your target.)

Just as the producers must have one BL scene per episode, I must also post that BL scene as well. lols

Hiroki wakes up the next morning to find a note from Nowaki saying he's gone. At work, Hiroki is teased by his co-worker Professor Miyagi, who-I-thought-he-was-gay-but-he-actually-has-an-ex-wife. Reminiscing about his relationship with Nowaki, Hiroki feels that it had been cold and distant. He sends Nowaki a letter, breaking up with him. Hiroki also moves, as well as changing his phone numbers.

Nowaki meets Hiroki at the university, though, and follows Hiroki even though the latter runs away. Over the course of walking a mile (Srsly, they walk for like forever.), Hiroki repeatedly refuses Nowaki while battling his own emotions. Barricading himself in his office, Hiroki tells Nowaki to go away. Professor Miyagi comes by as Nowaki is leaving, though, and Miyagi's friendly demeanor with Hiroki causes Nowaki to have all the wrong impressions, or so is implied.

After the ED, Misaki is unendingly tortured while Usami's teddy bear is out for cleaning. Usami uses the bear's absence as an excuse to have more intimate situations with Misaki.

Thoughts:
So haha, Hiroki tsun-tsuns throughout the entire episode. I guess I can understand why Hiroki feels tired of Nowaki's nonchalant attitude after six years, though. And wow six years? What kind of a timeskip is that? Since Hiroki and Nowaki's timeline appears to parallel Usami and Misaki's timeline, does that mean Misaki's been living with Usami for six years, too? Hrm. Nowaki's leaving of Hiroki without saying anything is pretty bad, though. The whole excuse, 'I was so busy I couldn't call you' is not one at all; if you really care about something you'll make time for it.
Lastly, I must say as terribly interesting as this parallel storyline is, I can't say I care too much for it. Focus on one couple, srsly. Not even harem animes do this.