Defender Kyle Bartley scored an extra-time volley to seal Birmingham's League Cup win over Plymouth.
Lewis Alessandra twice levelled for the League Two side, cancelling out Scott Allan's second-half brace.
Alessandra's injury-time strike to force the game into extra time came direct from a poor Bartley clearance.
Lee Novak failed to double the advantage after Allan's opener, firing a penalty straight at Argyle goalkeeper Jake Cole.
Cole also frustrated Novak just before the break, parrying his low 18-yard drive.
Alessandra was a constant menace for the visitors, while summer signings Romuald Boco and striker Marvin Morgan both threatened early on.
VIEW FROM THE DRESSING ROOM
Birmingham boss Lee Clark told BBC WM:
"It's not the way I would've liked it but thankfully we were victorious.
"I said to the players at half-time that we're on the verge of a shock here if we don't pull our finger out."
"We learnt a lot but we have to do better. The play was terrific at times but we can't just play when we've got the ball."