In part one Billy and Mandy find Nigel Planter hiding on a golf course, because he's been hunted by Lord Moldybutt, a wizard so terrible, the mere mention of his name causes things to break. After detailed investigation and several attacks on Nigel, Mandy discovers that the real culprit is not Lord Moldybutt at all. In part two Grim takes Billy and Mandy to the Underworld circus run by Dr. Clotho. The circus is on a verge of bankruptcy, so when Clotho notices Billy and Mandy in the audience, he decides to get them at all costs (in the Underworld real human children would be a sensation). Since Grim can't sell the kids (even though he would've loved to), Clotho kidnaps them, much to Grim's delight, until Billy's Mom forces Grim to get Billy back. Even so, Grim is reluctant to rescue the kids, so Mandy takes the matter into her own hands and ruins Clotho for good.
—Dimitry Rotstein