Plymouth's Ryan Hardie missed a penalty late in extra time as Chelsea survived a huge scare to beat the League One side in the FA Cup fourth round.
Macaulay Gillesphey's early header from Jordan Houghton's cross sent the 6,000 travelling Argyle fans into delirium.
Chelsea deservedly restored parity through Cesar Azpilicueta's delightful flick late in the first half.
Marcos Alonso put the Blues ahead in extra time before Kepa Arrizabalaga saved Hardie's tame spot-kick.
After registering 41 shots and 20 corners, Chelsea were worthy victors but were pushed all the way by a resilient and hard-working Plymouth.
Chelsea manager Thomas Tuchel was missing from the dugout after testing positive for Covid-19 on Friday, with assistant Arno Michels taking charge of the hosts.
Source: BBC