the next thing...

(the) next thing (one) knows

All of a sudden; out of nowhere; without warning. I was standing in line, minding my own business, when the next thing I know, a gunman runs into the bank and starts screaming at us to get on the ground! Tammy had only been on a few dates with Mark, but this one had been going smoothly. Next thing she knew, he got on one knee and proposed to her.
See also: know, next, thing

the ˈnext thing (I knew)...

(informal) used when somebody tells a story and wants to say that something happened suddenly or unexpectedly: I was just walking down the road and the next thing I knew someone was pointing a gun at my face.
See also: next