Tiny and Big: Grandpa's Leftovers

As the game opens, Tiny and the Radio are traveling by taxi into the "mysterious desert," where Tiny plans to confront Big over control of the Pants. The taxi is struck by an unknown object, leading the robotic driver to spin out of control and into a canyon. Tiny, unsettled by the crash but unscathed, collects his belongings from the crash site and attempts to continue on foot before being stopped by a cliff. Before getting the chance to think of an alternate way through, Big appears behind him, kicking him off of the cliff face.
Tiny lands in front of a massive statue, which he uses the interior and exterior of to climb upward. Along the way Big uses his Pants-powered ability to leap long distances to taunt him, leading for the entire structure to collapse behind the two. Big reveals his plan to enter a nearby pyramid to strengthen his powers, and Tiny and the Radio give chase while Big uses the Pants' telekinesis with increasingly deadly force to keep them at bay, lifting the pyramid into the sky as the chase goes on.
As Tiny and Radio pursue Big into the pyramid, they discover the origin of the Pants from the paintings on the walls of the pyramid. The Pants, which were once a normal pair of underwear, were struck by lightning and found by the Dotties, a race of mole-like creatures living in the ground; one of the creatures places the Pants on his head and gains superpowers from them, leading the others to worship him as a king. Under the eye of the leader, the Dotties are initially prosperous, forming a civilization with the pyramid and statue. Over time, though, the king is driven further into insanity by the Pants, eventually wiping out the entire population with their power.
Meanwhile, Big discovers the king's throne and sits atop it, reactivating a set of pyres within the pyramid. Fearing the possibilities of what might happen if Big keeps the Pants, Tiny and the Radio enter the king's lair and destroy the pyres, which causes the pyramid to collapse on itself. Big and Tiny have a final duel atop the floating pieces of the pyramid, which culminates in Tiny slicing a boulder over Big's head, incapacitating him.
As the wreckage of the pyramid lands, both boys wake up on the ground with the Pants laying near both over a Dottie hole. The two bicker over who their grandfather intended to be rightful owner of the pants, but before either gets the chance to grab them a Dottie surfaces from the ground and places them over its head. Knowing the entire process is going to happen again but lacking the item that drove their conflict, Big and Tiny form an unspoken truce, continuing to argue as they walk into the sunset together.