When originally aired on TV, the episodes were shown out of order. Even on IMDb, the episodes are sorted by their original air date not in the order the matches the overarching storyline. The proper viewing order is as follows:
Flames of Doom (1),
Escape from Ape City (2),
The Unearthly Prophecy (5),
Tunnel of Fear (4),
Lagoon of Peril (3),
Terror on Ice Mountain (12),
River of Flames (13),
Screaming Wings (6),
Trail to the Unknown (7),
Attack from the Clouds (8),
Mission of Mercy (9),
Invasion of the Underdwellers (10),
Battle of the Titans (11)