The other passenger - Grand Format

"You'll wish you'd never met me...". It all happens so quickly. One day you're commuting to work by river bus with your friend Kit in the seat beside you. The next, Kit hasn't turned up and his wife has reported him missing. At your stop, the police are waiting. They say another passenger saw you and Kit arguing on the boat home Last night. They say you have a reason to want him dead. You protest.
Who is this other passenger pointing the finger ? What do they know about your lives ? No, whatever danger followed you home Last night, you are innocent, totally innocent. Aren't you ?