Multiverse Explorer

Multiverse Explorer is about Ron Merritt, an ordinary human who befriends two visitors from another world and gets invited to visit them on their home planet. His new friends are members of an advanced race of humans from a world in another universe. Travel to that world is possible by using devices created by an ancient race of beings, who are surprisingly human in character but not remotely human in any other way. These aliens who created the travel-gate system disappeared many years ago.

During his visit, Ron develops a secret relationship with an infant from the alien Builder race. Working together, they manage to repair a broken travel gate, which leads to his being hired to work for the other world's Exploration Department.