Tenkai Dragon Cube / Secret of the Tenkai Dragon

S1, E2: After their first trip to Quarton, Mr. White and The Guardian fill them in on Quarton's history. Ceylan refuses to return to Quarton, but Guren insists they've been summoned to protect both worlds from destruction. During a visit to the museum, the two boys see an ancient stone tablet that later turns into a key in front of them.