The Secret Oath (The Rugendo Rhinos Series, Book 4) by Arensen, Shel

Exotic, edge-of-your-seat mysteries pre-teen boys will devour. Matt, Dean, Dave, and Jon are members of the Rugendo Rhinos club. In Rugendo, Kenya, where they live with their missionary parents, these four are never far from an adventure.

When the Rhinos accidentally stumble across a secret African oathing ceremony, they know they must forget what they’ve seen . . . or else. Before they know what’s happening, Matt is kidnapped right under their noses. Once again, it’s up to the Rhinos and Cheetahs to discover what happened and rescue their friend.