Mystery of the Missing Sister — Jerry B. Jenkins

Dallas O'Neil and his Baker Street friends are back! And now, besides playing all their favorite sports, they are solving exciting and sometimes dangerous mysteries. -- One of the most popular Baker Street kids is Jack Bastable, a gentle giant who is mentally retarded. He's a friendly, harmless boy with a sweet spirit and unusual athletic talent. Or is he?

When his little sister disappears, the suspicion turns toward Jack. He's hurt and confused, but the evidence convinces even some of his friends on Baker Street.

He feels he has nowhere to turn, no one to talk to. Even Dallas and Jimmy are surprised and confused by how guilty Jack appears, but they will not turn against him until they are convinced.