Of American-English origin, the phrase this is where we came in, and variants, mean we are back at the point at which we started. It is used of discussions or arguments that are inconclusive and seem to go round in circles.

The reference is to cinema-going: when films were screened several times over, it was possible for spectators to go in and start watching a film at any point, staying on to watch the first part on its next showing, up to the point where they had come in—at which stage spectators would say to each other “This is where we came in. Let’s go”.

Russell Channing ‘Russ’ Westover (1886-1966) evoked those consecutive screenings—and made Tillie use the phrase—in the following episode of the cartoon strip Tillie the Toiler, published in the Corsicana Daily Sun (Corsicana, Texas) of Thursday 5^th December 1935—Tillie and Mac are at the cinema:

A Help to Hate

– Let’s go, Mac – Here’s where we came in
– Wait, Tillie – Let’s see it over again?

– Goodness, Mac – Why did you want to sit thru that picture twice? – I thought that actor was your special hate
– He is – Tillie – –
– But I can hate him better when I’m lookin’ at him!

The earliest instance of this is where we came in that I have found is from the last of the unconnected paragraphs making up Roundabouts, “edited by A. Roundabouter”, published in The San Diego Union (San Diego, California) of Thursday 20^th September 1923—the fact that the author introduces the phrase with “As folks say at the movie shows” seems to indicate that this is where we came in was already used figuratively:

A modern wife doesn’t mind having a canary around the house, but there is no welcome sign out for the stork.
                                                                      ● ● ●
As folks say at the movie shows:
“This is where we came in. Let’s go.”

The following anecdote is from The Kansas City Star (Kansas City, Missouri) of Thursday 27^th June 1929:

AND THEN SHE GLARED.
A Woman’s “Most Embarrassing Moment” in a Downtown Theater.

There was a lull in the speech of the actors in a talkie at a downtown theater, so the house was quiet except for the soft orchestration of the melody of the theme song.
A man near the center of the house said to his woman companion:
“This is where we came in. Let’s go.”
He arose abruptly. Then, struck by an afterthought, he spoke in a voice clearly audible for several rows:
“Pardon me, dear; I haven’t given you time to put on your shoes.”

The phrase is used figuratively in one of the unconnected paragraphs making up Senator Soaper Says, published in the Morning Oregonian (Portland, Oregon) of Tuesday 19^th December 1933—the reference is to the battle of the Campo Vía pocket, a successful Paraguayan offensive during the Chaco War (September 1932 – June 1935) between Paraguay and Bolivia:

We gather from the sport columns that the railroads are not alone in scrapping wooden coaches.
                                                                      ● ● ●
It was the Paraguayans’ turn this month to take a battle. We shall leave the picture here, as this is where we came in.
                                                                      ● ● ●
An off-key voice is invaluable in any band of strolling carolers at Yuletide, as a householder will pay them more to hurry on.