Complete the Sector Beta mission to unlock the first set of portals. Then play through the game until you complete the mission on Fichina to unlock the Gravmaster transformation for the Landmaster tank. Next, return to Titania and play until you reach the portal area. Use the Gravmaster to ascend toward the portal, triggering a cutscene (don't miss the medal that is visible, just before the scene commences). After the scene concludes, you'll find yourself controlling Peppy in a special mission. Make sure that you perform some barrel rolls to hear how much your ally loves the maneuver.