The Secret of Two Generations - S1.E24

The gang are at Metz's place where Lok shows him the ancient Amulet of Will, as well as the legendary titan, Tao. It is then when Metz tells him about himself, Eathon Lambert, and another friend, Simon Judeau. Those three had been inseparable and had searched for many titans together. However, during one mission Simon disappeared. The gang believes that Simon might have the last legendary titan. But after some investigation, they make a shocking discovery concerning the Professor.