Phineas was so glad that Perry was there with them - the weekend previous, Ferb, Isabella and he constructed a transporter on the roof of their dorm that allowed them to go visit their parents and finally point out that yes, Phineas FINALLY asked Isabella out. This resulted in Phineas and Ferb's mom going "FINALLY!" and Mrs. Garcia-Shapiro to nearly crush the ribs of Phineas in a hug, before, as usual, she offered him food. But the end of the visit was bittersweet - they wouldn't get another chance to see their parents until break rolled around, and as mostly a last-minute consideration of the boys' father, they sent along the family pet, Perry. "He's been looking really lonely without you boys around," Linda explained, and Phineas smiled fondly as he held Perry over his shoulder, as usual. "Don't worry, I'm sure there's a lot of stuff for him not to do at college," Phineas joked, and with a bag of Perry's belongings, they returned to campus with platypus in tow.
Of course, Phineas and Ferb knew something about their pet that very few others knew. Even Perry didn't know that they knew, but somewhere along the lines, both brothers had figured it out - Perry was an Agent for some company that was bent on keeping evil at bay. Phineas wondered about how that would affect Perry's ability to come along with - if perhaps they should KEEP the transporter up there, on the roof, in case Perry needed to thwart... well, thwart whatever it was platypi thwarted when they weren't at home doing nothing.
So it's been about a week since they went home. Phineas was sitting on the floor, working on an essay on his laptop while idly watching TV, and chewing on his thumbnail in concentration. It was really hard to concentrate, considering he was worried about his longest-had friend, and what kind of plans the platypus had, what he was thinking under that dumb, walleyed exterior.