this is where I came in

Fig. I have heard all this before. (Said when a situation begins to seem repetitive, as when a film one has seen part of before reaches familiar scenes.) John sat through a few minutes of the argument, and when Tom and Alice kept saying the same thing over and over John said, "This is where I came in," and left the room. The speaker stood up and asked again for a new vote on the proposal. "This is where I came in," muttered Jane as she headed for the door.

This is where I began, my knowledge dates from this point. For example, Do you have anything more to add, because if not, this is where I came in. This idiom, dating from the 1920s, originally alluded to the continuous showing of a motion picture, with customers entering the theater at any stage while the film was running and leaving when it reached the point where they had started.