Now You See Me

Six people went in. Only one came out...
Introducing Standedge Tunnel: the longest canal tunnel in England.
Last year six students went in, and two and a half hours later, the boat reappeared on the other side with only one of the students, unconscious, and the dog.
The case of the Standedge Six was largely kept from the national media. The police investigation concluded that the only remaining student, Matthew, killed his friends, hid the bodies on the boat and returned later to move them to an undisclosed location.
Matthew is in prison . . . but maintains he is innocent.
Robert Ferringham is grieving for his missing wife, Sam. So when Matthew contacts him for help with his case, promising information on Sam, Robert has no choice but to help. But can he trust Matthew?
And how will he solve the insolvable case?