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."
Birmingham City 13 Doyle 02 Eardley Booked 05 Bartley 06 Burn Booked 27 Ferguson (Lee - 90' ) 17 Reilly 24 Adeyemi (Elliott - 77' ) 26 Allan 07 Burke 12 Novak 16 Arthur Booked (Robinson - 71' ) Substitutes 01 Randolph 04 Robinson 08 Mullins 15 Elliott 20 Lee 22 Shinnie 25 Green Plymouth Argyle 01 Cole 02 Berry 03 Blackman 16 Trotman 17 Nelson 06 Hourihane 07 Alessandra Booked 08 Boco (Vassel - 88' ) 11 Blizzard (Young - 106' ) 09 Reid (Harvey - 78' ) 10 Morgan Substitutes 23 McCormick 22 Richards 25 Purrington 14 Young 30 Lane 18 Harvey 24 Vassel