Trials of Lion-O, Part 1 is the fifteenth episode of ThunderCats 2011 TV series.
Lion-O and crew follow the Book of Omens to a tall cliff, where the book tells them to go up. As the group go up a trail, the others begin to make comments about the height and how exposed the position is. Shortly after stopping for a break, the group is attacked.
Slithe manages to get the sword and the gauntlet, but Lion-O manages to get the gauntlet back after Slithe threatens to kill WilyKit. Lion-O finds himself on the cliff's edge, where he is pushed into the river by Kaynar and Addicus. Slithe then states that they will have to go down to the river to retrieve the body and the gauntlet.
Meanwhile, Lion-O , trapped on the bottom of the river by some boulders, drowns and finds himself in the Spirit World and he comes face to face with Jaga's ghost. Jaga informs Lion-O that the Spirit Stone found him worthy of a second chance at life...if he can pass a series of trials. Lion-O accepts and is whisked away to his first trial. He is confronted by two spirits taking the shape of the Wily Twins. They inform him that he has to steal a key from them in order to move onto the next trial. But the twins can create a near infinite amount of holograms, causing Lion-O confusion. Eventually he realizes he can use his sense of smell to weed out the fakes. As the twins lament that Lion-O is taking too long, Lion-O informs them that he already has the key. Impressed, the twins send him off to the next trial.
Back at the lizard army, the captured Cats mourn the death of Lion-O. They look to Tygra, the new heir to the throne, for leadership. He goads Slithe and Addicus until Addicus attacks him. The twins use the opportunity to free themselves and the rest of the team, who all attack. But as they are fighting, Slithe orders them to fall back, as he has the Sword of Omens, and that's all Mumm-Ra will care about. So they leave the Cats, as Tygra vows revenge and to get the sword back.
Lion-O enters his second trial. It's a race through a labyrinth against a spirit disguised as Cheetara. The first one to the center, where the key is, wins. He is given a head start and races from one dead end to another. As Cheetara eventually passes him, Lion-O falls back, giving up. He sees a Flying Creature and realizes he can just use that to fly to the key, which he does. He barely beats Cheetara to the key. Thinking he has won, he is informed that he is only halfway through the trials.
Back in the land of the Living, the surviving ThunderCats fight to free themselves from captivity.
- Lion-O's first two trials take place in realms resembling the slums of Thundera and the Briar Forest of the Petalars.
- The trials Lion-O faces are similar to the Anointment Trials from the Original series, mainly Lion-O's Anointment Second Day: The Trial of Speed where he has to race against Cheetara and Lion-O's Anointment Third Day: Trial of Cunning where he has to outwit the Thunderkittens