Passenger door won't open

Passenger door has always been temperamental but now it's given up. Handle is saggy and doesn't do anything (inside or outside)

Luckily we've got no door cards etc so can see what's going on... The plastic nut shaped thing has come out of the hole which would then pull the latch. We can push it back in but as soon as the handle is pulled it pops back out.

Tried to look in the drivers side to compare but it looks completely different (maybe the passenger one has been uncovered and, apparently, broken).

If it's easy enough to fix, I'll attempt that. But if anyone has any tips for just getting the thing open, that'll be great too!