Don't you hate it when you're sitting on the toilet and some guy runs in and strangles you to death?
That was the situation Rick found him in as a group of roadwarrior-wannabes wandered into the home he, Carl and Michonne had claimed and started spitting and acting like drunk frat boys.
Glenn is now about 3 hours further down the road than Maggie, and finds himself with Sgt. Abraham Ford, whats-her-name from the Gov's camp, a "scientist" that looks and acts more like Rainman with a mullet than a Rhodes Scholar, and some hispanic chick with a chip on her shoulder and the shortest shorts of the apocalypse (lucky for her she didn't run into the guys that Rick did).
So, now Glenn's on his way back to find Maggie (on foot after previously-mentioned scientist shot up their truck trying to stop the zombie children of the corn), Rick had to sneak his way out of the house Bond-style, Carl finds out a little more about Michonne's life before the apocalypse, and Michonne finds the creepiest suicide room seen since season one!
OH! And, somehow, backwoods genius, Eugene the scientist knows how the apocalypse happened and how to fix it (but it's classified of course)!
Here... I'll let them explain:
Aim high. Run fast.